In order to support cases where pools and keys are pre-created and
ceph-proxy just proxies this data to client applications this change
introduces support for:

* having custom "admin" users which may not actually have admin
privileges on the target cluster (client.admin is probably occupied by
real admins in this case);
* using cephx keys provided via charm config.

Change-Id: I01014b6986f92bf0ad8147a08afa1d61fdd5c088
Closes-bug: #1793991