Report distribution.version.modifier fact. ex 'beta'

By default, if it exists, parse /etc/os-release.  This file
is more general than redhat-release, and contains data
that doesn't require us to parse it.

If /etc/os-release doesn't exist, parse /etc/redhat-release
slightly modified to pick up version modifiers (beta, for
example)

If there is no redhat-release file, we fallback to the
platform module, which doesn't have the ability to detect
beta versions, however it should work on other operating
systems.  I'm not sure how necessary the platform module
is at this point.
Merge pull request #808 from candlepin/ckozak/detect_beta

report version modifier