mgr: re-enable mds `scrub status` info in ceph status

mgr: re-enable mds `scrub status` info in ceph status

MDS does not register with manager, therefore `task status`
(scrub status updates) were not getting displayed in ceph
status output.

Signed-off-by: Venky Shankar <vshankar@redhat.com>
(cherry picked from commit d6ad0a255a27c5e3da20c7ae03066a692a6d49bb)

 Conflicts:
	src/mgr/DaemonServer.cc
	src/mgr/DaemonServer.h

mgr/DaemonServer.{h,cc} deals with raw pointers while master uses ref_t<>
cast -- adjust to that. a minor conflict in the header and the metrics
templatization is not backported to nautilus. also, DaemonKey is a std::pair
in nautilus but a struct in master -- that requires a change in referencing
daemon type and name.