This change will add two new actions to the ceph-mon charm. These
actions will allow the user to create and remove cache tiers from
existing pools.  Both writeback and read only mode are supported.

Limitations of this patch include not having fine grain control
over the cache tier properties.  Things like hit_set_count,
bloom filter control or cache sizing are not supported yet.

Change-Id: I5a37e79d0d23d35295a8ae97177c940af66b0485