use variables for release numbers in upgrade tests
    
    Summary:
    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".
    
    Test Plan:
    Apply with D702, schedule upgrade tests, and make
    sure the correct hard disk image filenames are used.
    
    Reviewers: jskladan, garretraziel
    
    Reviewed By: garretraziel
    
    Subscribers: tflink
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D703