* Use the correct keyring for the OS (a refactor of the changes suggested in #41)

updated repositories entries for sane naming across Debian and Ubuntu entries
updated repo entries for security as syntax differs between Debian and Ubuntu
added repo entries for -updates (previously known as volatile)
updated map to use explicitly use grain os instead of implicitly use os_family
removed distribution_updates variable - syntax was Debian specific and invailid for Ubuntu
tested against Debian/stretch and Ubuntu/xenial

remove_* was recently changed to always place empty file, but just set
perms if not present, without replace.

Do the same with that option.

- Add template file
- Add pillar example

[WARNING ] State for file: /etc/apt/preferences - Neither 'source' nor
'contents' nor 'contents_pillar' nor 'contents_grains' was defined, yet
'replace' was set to 'True'. As there is no source to replace the file
with, 'replace' has been set to 'False' to avoid reading the file
unnecessarily.

Now, if file.managed, but when content is not changed (no source,
contents, ...), avoid the warning.

contributed initialy by @javierbertoli <javier@netmanagers.com.ar>

fixes #30

Fixes #30

Deleting the file causes "could not open file '/etc/apt/sources.list'"
warnings. Better to keep it around but just remove its contents.

Salt uses the aptsources system library to get a list of repositories on
the host. Unfortunately if the /etc/apt/sources.list file is missing,
aptsources.sourceslist.SourcesList() (which is called by salt) logs a
warning, which is then displayed to the user running salt.

To avoid this warning when using 'remove_sources_list: true' in pillar
we can keep the /etc/apt/sources.list file, but just ensure it is empty.
This achieves the same thing, but does not log the warning or display it
to the user.

Ignore .kitchen dir

Add defaults for preferences mgmt

Add some pillar examples for preferences mgmt

Add tests for preferences

Add entries in the README file

Missing map.jinja entry