Removed dependency for ``cinder_backends_rbd_inuse`` in nova.conf
when setting ``rbd_user`` and ``rbd_secret_uuid`` variables.
Cinder delivers all necessary values via RPC when attaching the
volume, so those variables are only necessary for ephemeral
disks stored in Ceph. These variables are required to be set up
on cinder-volume side under backend section.
Change-Id: If505dc58b4adb46e08230b0482d48d8898529460
Closes-Bug: #1673570