Don't add key_url as it is deprecated and not needed when
pkgrepo_keyring is set. This has been supported since stretch so make it
the default. Also use py3 repo by default for Debian family as it is now
the only option.

Additionally, Raspbian has been updated to use signed-by by default.

Also:

* add keyring file for the salt repo, due to `apt-key` deprecation
* remove old repository file

Use the following command:

```
$ GREP_SED="\(repo\.salt\)stack\.com" \
  && grep -rl --exclude-dir=.git "${GREP_SED}" \
  | grep -v .un~$ \
  | xargs sed -i -e "/${GREP_SED}/s//\1project.io/g"
```

Fix #495

Make the upstream salt package repository selectable, thus allowing the
use of archived salt versions (hosted in
https://archive.repo.saltproject.io), as well as custom salt versions
hosted in alternate repositories.

* add `-` if someone supplies `py3` in pillar, like other platforms

* Semi-automated using https://github.com/myii/ssf-formula/pull/31
* Includes:
  - Update TOFS
  - Use `bin/kitchen`
  - Use `dist: bionic` in Travis
  - Add `yamllint` and apply rules
  - Use `pillars_from_files` throughout
  - Replace EOL images in Kitchen & Travis
  - Add `develop` images in Kitchen & Travis
* Fix (or ignore)  errors shown below:

```bash
salt-formula$ yamllint -s .
./pillar.example
  1:1       warning  missing document start "---"  (document-start)
  5:23      warning  truthy value should be one of [false, true]  (truthy)
  8:25      warning  truthy value should be one of [false, true]  (truthy)
  11:25     warning  truthy value should be one of [false, true]  (truthy)
  19:21     warning  truthy value should be one of [false, true]  (truthy)
  40:89     error    line too long (108 > 88 characters)  (line-length)
  41:89     error    line too long (112 > 88 characters)  (line-length)
  43:89     error    line too long (112 > 88 characters)  (line-length)
  45:89     error    line too long (110 > 88 characters)  (line-length)
  47:89     error    line too long (89 > 88 characters)  (line-length)
  74:27     warning  truthy value should be one of [false, true]  (truthy)
  82:9      error    wrong indentation: expected 10 but found 8  (indentation)
  101:14    warning  truthy value should be one of [false, true]  (truthy)
  102:20    warning  truthy value should be one of [false, true]  (truthy)
  103:89    error    line too long (119 > 88 characters)  (line-length)
  121:7     warning  comment not indented like content  (comments-indentation)
  122:24    error    syntax error: found character '%' that cannot start any token
  310:89    error    line too long (102 > 88 characters)  (line-length)
  330:89    error    line too long (113 > 88 characters)  (line-length)
  433:1     error    too many blank lines (1 > 0)  (empty-lines)

./salt/osmap.yaml
  4:2       error    syntax error: found character '%' that cannot start any token
  6:89      error    line too long (93 > 88 characters)  (line-length)
  22:89     error    line too long (137 > 88 characters)  (line-length)
  23:89     error    line too long (134 > 88 characters)  (line-length)
  33:89     error    line too long (149 > 88 characters)  (line-length)
  34:89     error    line too long (146 > 88 characters)  (line-length)

./salt/osfamilymap.yaml
  4:2       error    syntax error: found character '%' that cannot start any token
  6:89      error    line too long (94 > 88 characters)  (line-length)
  24:89     error    line too long (149 > 88 characters)  (line-length)
  25:89     error    line too long (146 > 88 characters)  (line-length)
  39:89     error    line too long (105 > 88 characters)  (line-length)
  40:89     error    line too long (127 > 88 characters)  (line-length)
  56:89     error    line too long (101 > 88 characters)  (line-length)

./salt/ospyvermap.yaml
  4:1       warning  missing document start "---"  (document-start)

./salt/defaults.yaml
  3:1       warning  missing document start "---"  (document-start)
  7:21      warning  truthy value should be one of [false, true]  (truthy)
  8:12      warning  truthy value should be one of [false, true]  (truthy)
  9:23      warning  truthy value should be one of [false, true]  (truthy)
  10:19     warning  truthy value should be one of [false, true]  (truthy)
  14:25     warning  truthy value should be one of [false, true]  (truthy)
  15:25     warning  truthy value should be one of [false, true]  (truthy)
  16:27     warning  truthy value should be one of [false, true]  (truthy)
  17:27     warning  truthy value should be one of [false, true]  (truthy)
  39:28     warning  truthy value should be one of [false, true]  (truthy)
  41:28     warning  truthy value should be one of [false, true]  (truthy)
  45:24     warning  truthy value should be one of [false, true]  (truthy)
  49:30     warning  truthy value should be one of [false, true]  (truthy)
  54:28     warning  truthy value should be one of [false, true]  (truthy)
  63:25     warning  truthy value should be one of [false, true]  (truthy)
  68:15     warning  truthy value should be one of [false, true]  (truthy)
```

* From `salt-winrepo-ng`:
  + https://github.com/saltstack/salt-winrepo-ng/blob/master/salt-minion-py2.sls
    - Package: `salt-minion`
  + https://github.com/saltstack/salt-winrepo-ng/blob/master/salt-minion-py3.sls
    - Package: `salt-minion-py3`
* Close #411

* Prepare for selecting correct version of Windows `salt-minion` pkg

Use py3 for latest salt version
Use Python 2 on old distros
Update Gemfile
Updated centos-6 run_command, as it does not use systemd
Added tests to verify correct package versions for all cases tested

e.g. TurnKey Linux amongst others