Support client creation with pre-generated key

[scarvalhojr]

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.

[scarvalhojr]

Any interest in getting this merged, @guilhem?

[guilhem]

@scarvalhojr I mainly have to review it and it's quite huge ;)

[hufman]

Looks good to me!