Process requests for radosgw per unit keys. Ideally the charm
would move to using charms.ceph.utils for the get_radosgw_key
method but this does not work atm. I have raised bug #1899643
to cover deduplicating hooks.ceph v charms.ceph.*

Change-Id: I00f5a58f127baa1f7878b0ddb31b4fa009d9651e
Closes-Bug: #1899634