Along with the matching change to fedora-openqa-schedule to pass
these variables in when scheduling jobs, this avoids hardcoding
the release numbers for the upgrade tests (which means someone
has to remember to edit them every release). The new createhdds
similarly uses get_current_release() to decide what releases it
needs images for, so all this should hook up and work magically
without any human intervention required.
For clarity, the effect of the '_upgrade_' tests is "run an
upgrade from the 'current' Fedora release to whatever release
is being tested", and the effect of the '_upgrade_2_' tests is
"run an upgrade from the 'previous' Fedora release to whatever
release is being tested".