When the Ceph user is restricted to a specific namespace in the pool, it is
crucial that evey interaction with the cluster is done within that namespace.
This wasn't the case in `getCloneDepth()`.
This issue was causing snapshot creation to fail with
> Failed to check and update snapshot content: failed to take snapshot of the
> volume X: "rpc error: code = Internal desc = rbd: ret=-1, Operation not
> permitted"
Signed-off-by: Benoît Knecht <bknecht@protonmail.ch>