As of Ocata, the ceph key used to access a specific Cinder
Ceph backend must match the name of the key used by cinder,
with an appropriate secret configured for libvirt use with
the cephx key used by the cinder-ceph charm.
Add support for the new ceph-access relation to allow
nova-compute units to communicate with multiple ceph
backends using different cephx keys and user names.
The lead cinder-ceph unit will generate a UUID for use in
the cinder configuration file, and for use by the remote
nova-compute units when configuring libvirt secrets,
ensuring that both ends of the integration match up.
The side effect of this change is that nova-compute will
have a key for use with its own ephemeral backend ceph
access, and a key for each cinder ceph backend configured
in the deployment.
(cherry picked from commit 62613456e7a04ac348d4589145a550323dfa5a55)