This avoids problems when values are strings containing colons. And it
mimicks what was already done for the salt-minion's configuration file.

Fixes #233.

- When it's iterable, the minion could be running on the master
- When it's a string, there's no advantage over just specifying
  `salt:minion:master`

This makes it easier for users to check the status of jobs that ran into
a timeout.

With a simple pillar like this::

    $ sudo salt-call --config-dir /srv/etc/bootstrap --pillar-root /srv/pillar pillar.get salt:pillar_roots
    local:
        ----------
        base:
            - /srv/pillar

This was generated in /etc/salt/master.d/f_defaults.conf::

    # highstate format, and is generally just key/value pairs.
    pillar_roots:base:- /srv/pillar
    #

Resulting in parse errors by salt::

    $ sudo salt '*' state.highstate
    [ERROR   ] Error parsing configuration file: /etc/salt/master.d/f_defaults.conf - while scanning a simple key
      in "<string>", line 531, column 1:
        pillar_roots:base:- /srv/pillar
        ^
    could not found expected ':'
      in "<string>", line 532, column 1:
        #
        ^
    [ERROR   ] Error parsing configuration file: /etc/salt/master.d/f_defaults.conf - while scanning a simple key
      in "<string>", line 531, column 1:
        pillar_roots:base:- /srv/pillar
        ^
    could not found expected ':'
      in "<string>", line 532, column 1:
        #
        ^

This patch will fix it as such::

          ID: salt-master
    Function: file.recurse
        Name: /etc/salt/master.d
      Result: True
     Comment: Recursively updated /etc/salt/master.d
     Started: 11:37:12.946823
    Duration: 6255.296 ms
     Changes:
              ----------
              /etc/salt/master.d/f_defaults.conf:
                  ----------
                  diff:
                      ---
                      +++
                      @@ -528,7 +528,9 @@
                       # Pillar is laid out in the same fashion as the file server, with environments,
                       # a top file and sls files. However, pillar data does not need to be in the
                       # highstate format, and is generally just key/value pairs.
                      -pillar_roots:base:- /srv/pillar
                      +pillar_roots:
                      +  base:
                      +    - /srv/pillar
                       #

Resulting in::

    # highstate format, and is generally just key/value pairs.
    pillar_roots:
      base:
        - /srv/pillar
    #

This commit fixes how `pillar_roots` are generated and after this fix the
generated configuration does not contain any unnecessary new lines:

```yaml
pillar_roots:
  base:
     /srv/salt/dir1
  dev:
     /srv/salt/dir2
     /srv/salt/dir3
  locale:
     /srv/salt/dir4
```

Before this commit the pillar_roots in `f_defaults.conf` for master would be
generated with a lot of empty lines in between directories, like this:

```yaml
pillar_roots:

  base:

     /srv/salt/dir1

  dev:

     /srv/salt/dir2

     /srv/salt/dir3

  local:

     /srv/salt/dir4

```

The minion configuration is not affected and renders fine.

The user will already have it's /etc/salt/minion file, so it doesn't need all this info, and it makes easier to know what has been generated and what not

Needed because salt-cloud will attempt to load them even if they are
filled with invalid default values, creating error spam.

Added config setting for:

* pillar_source_merging_strategy

This obsoletes the salt:cloud:folders and salt:cloud:providers pillar
entries. Provider keys have been moved to /etc/salt/pki/cloud.

These aren't intended to function; they're here to allow the use of
file.recurse on the provider folder, without requiring the user
to provide pillar data for templates they're not using.

Salt writes it's schedule file to /etc/salt/{minion,master}.d/_schedule.conf

We don't want to stomp all over Salt's files, but we do want a pristine
starting point to lay down our managed config. So we use clean: True on the
file.recurse call, but we tell it to ignore files that start with an _

We have to rename the current config file (_defaults.conf) because it will be
ignored by the rule that ignores Salt's _* config files.

This also means we need to clean up old config files (_defaults.conf) and
restart the service if we cleaned it up.

This will fix #39

And it will be backwards compatible with the current pillar configuration.

see pillar.example for configuration

This is necessary for the correct settings on the minions when saltifying new hosts

If those options are set in pillar data, the jinja template
salt/files/master.d/_defaults.conf would fail to compile trying to
evaluate non-existing variables.

Replace those variables with the corresponding dictionnary entries.

This is only needed for salt versions older than 2014.7 that do not
define the global "salt" variable in such context.