systemd/ceph-mgr: remove automagic mgr creation hack

systemd/ceph-mgr: remove automagic mgr creation hack

For kraken we auto-created mgr daemons next to mon daemons with some
systemd hackery.  This is awkward (you can't not get a new mgr daemon when
you deploy a mon), systemd-specific (not implemented for upstart on
trusty), and mostly unexpected.  Since ceph-mgr daemons are now first-class
citizens and required for every cluster, make their deployment explicit
and transparent to the administrator.  Major upgrades are a rare
opportunity to have the administrator's full attention so take advantage
of it.

This effectively reverts 61d779345e9efbe9a2e3f215af1f1dcf6630f04a and
082199f69dd0bd4c18a5f4baea67a88782586657 (and follow-on fixes).

Fixes/avoids: http://tracker.ceph.com/issues/19994
Signed-off-by: Sage Weil <sage@redhat.com>