copr-be: reuse opportunity time decrease
    
    This will recycle VMs more quickly, after 90s of idling.  This is giving
    their assigned users smaller changes to re-take it.  This though should
    not be a problem nowadays since copr-backend is pretty optimal and each
    "task dispatching cycle" takes less than 30s.  If users have multiple
    tasks to process, these tasks should still be happily dispatched within
    the time-frame.