mon: tests for ceph-mon --mkfs

* auth none must not require a keyring
* --key can be used as an alternative to --keyring
* --mkfs is idempotent
* the --mon-data directory is created if it does not exist
* auth ceph requires --keyring

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1012 from dachary/wip-mon-mkfs

unit tests for ceph-mon --mkfs