mkcephfs.in: print out usage if no actions given
mkcephfs.in: print out usage if no actions given

If the user didn't specify any actions, print out a usage message rather
than silently exiting.

Signed-off-by: Colin McCabe <7b35906f1d014e74a9dc8c913eb229119df647aa@dreamhost.com>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph