(maint) remove operatingsystemrelease duplication

Prior to this commit, the `operatingsystemrelease` fact had a
significant amount of duplicate code in determining the release version
of the operating system.
This commit condenses that code to a more managable format.
(maint) fix formatting and add more testing info

Prior to this commit, there were some formatting issues taking away from
readability of `lib/facter/operatingsystemrelease.rb`, and there was
limiting testing of this file. This commit fixes those formatting
issues, and adds more test coverage.
(maint) remove whitespace and alter formatting

Prior to this commit there were some whitespace errors in the code,
relevant information for the test_cases was stored in two different data
sets, and there was unnecessary comments. This commit fixes the
whitespace issues, reformats the datastructure of the test_cases hash,
and gets rid of the comments.
Test operatingsystemrelease output where possible

On platforms where we have fixture data available for the
operatingsystemrelease fact, we should test the parsing of the actual
files. On other platforms where we lack such fixtures, just ensure that
the correct file is read.

Paired-with: Melissa <melissa@puppetlabs.com>
Merge branch 'pull-450'

This closes GH-450.