Relation data isn't always updated on config-change

The subordinate_configuration in the storage-backend relation doesn't update sometimes.

I've encountered several situations where the configuration in /etc/cinder/cinder.conf doesn't match the configuration of the cinder-ceph charm.

One scenario:

- Deploy cinder-ceph related to cinder using rbd-pool-name=thepool.
- Create a new pool with a name "different".
- juju conifg cinder-ceph rbd-pool-name=different

The cinder.conf will continue to have "thepool".

I believe what is happening is the ceph_changed function (which is called as a part of config-changed) only updates relation data when it successfully creates the pool.

I've had other scenarios where the config file didn't get updated but this is the only one I've been able to nail down.