debian: add start ceph-mds-all on ceph-mds install

This ensures that when we then start individual mds instances, we can
stop ceph-mds-all and they will get stopped.  We do the same already for
ceph-all.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 41897fcba168b409fd370d0b26016fe4d30c5a59)