mgr/volumes: Optionally authorize existing auth-ids

mgr/volumes: Optionally authorize existing auth-ids

Optionally allow authorizing auth-ids not created by mgr plugin
via the option 'allow_existing_id'. This can help existing deployers
of manila to disallow/allow authorization of pre-created auth IDs
via a manila driver config that sets 'allow_existing_id' to False/True.

Fixes: https://tracker.ceph.com/issues/44931
Signed-off-by: Kotresh HR <khiremat@redhat.com>
(cherry picked from commit 713270d1869e2370b674b1a4bc6f6a37023a5917)