Support client creation with pre-generated key

Support client creation with pre-generated key

Extend the client LWRP to allow a Ceph auth entity to be created with
a given key, instead of letting ceph auth generate a random one. This
is useful when the keys are managed elsewhere and are known beforehand.

Also, change the way an entity is recreated in case of a caps mismatch.
Instead of relying on parsing an error message from auth get-or-create,
explicitly check if entity exists and matches specification.