Legacy backup service support was recently dropped from cinder in
change I3ada2dee1857074746b1893b82dd5f6641c6e579 and we need to
adjust how we set the config option in devstack accordingly. This
updates the backup_driver option to specify a full class name instead
of only the module name.
Closes-Bug: #1794859
Change-Id: I3a72f38b564b8b83b233fccba7685833b6394d45