Add support for 'add-permissions-to-key' requests

Currently add-permissions-to-key requests are not being handled by this
charm, so if cinder-ceph or glance is deployed with
restrict-ceph-pools=True, it will not be configured properly. This
patch ports the implementation of the add-permissions-to-key
handling from the ceph-mon charm, and updates other methods required
to properly set up cinder-ceph and glance ceph pools.

Change-Id: If5950af155e10ee4d140300fd7ed5730798b65f0
Closes-bug: #1836408