mon: do not use the keyring if auth = none

mon: do not use the keyring if auth = none

The Monitor::is_keyring_required() predicate is defined to be used in
the mkfs code path and not require the keyring to be set unless it is
actually going to be used.

If authentication is turned on later on existing monitors, the keyring
files will have to be set by an external tool in the mon data directory
and the mon restarted.

Signed-off-by: Loic Dachary <loic@dachary.org>