pybind: ceph_volume_client authentication metadata

Store a two-way mapping between auth IDs and volumes.

Enables us to record some metadata on auth ids (which
openstack tenant created it) so that we can avoid exposing
keys to other tenants who try to use the same ceph
auth id.

Enables us to expose the list of which auth ids have access
to a volume, so that Manila's update_access() can be
implemented efficiently.

DNM: see TODOs inline.

Fixes: http://tracker.ceph.com/issues/15615

Signed-off-by: John Spray <john.spray@redhat.com>