ceph::mon : control client.admin key generation

Creating an empty /etc/ceph/ceph.client.admin.keyring file disables the
automatic creation of the client.admin credentials when the first
monitor is created.

The creation of the key must be done via ceph::key, using the
mon. keyring, in the same way ceph-create-keys does.

Modify the client.volumes test to actually use cephx instead of
injecting a key in a non cephx cluster. It makes the client.something
test redundant and also demonstrates that the client.admin key can be
created by puppet instead of being automatically created by
ceph-create-keys.

Change-Id: I7060e418c27ea3f6f4e53604044f21865438382e
Signed-off-by: Loic Dachary <loic@dachary.org>