Revise release variable handling, prerelease checks, os-release
I started out trying to fix os-release for the recent change to
add "Prerelease" tags to the VERSION and PRETTY_NAME fields, then
things spiralled. It got me thinking about the awkward DEVELOPMENT
variable we use, so I decided to get rid of it and refactor the
few things that use it. I refactored the anaconda prerelease tag
check, and wrote a new giant comment that gives details about
exactly how anaconda decides whether to show those tags, to give
context to our choices about when to expect them. This check now
uses a new LABEL variable the scheduler now sets. I also wound up
creating new UP1REL and UP2REL vars to define the 'source' release
for upgrade tests, separate from CURRREL and PREVREL, which are
now never lies - they really are the current stable and previous
stable release, even for update upgrade tests.
Signed-off-by: Adam Williamson <awilliam@redhat.com>