mon/OSDMonitor: make down osd count sensible

We currently log something like

 1/10 in osds are down

in the health warning when there are down OSDs, but this is based on a
comparison of the number of up vs the number of in osds, and makes no sense
when there are up osds that are not in.

Instead, count only the number OSDs that are both down and in (relative to
the total number of OSDs in) and warn about that.  This means that, if a
disk fails, and we mark it out, and the cluster fully repairs itself, it
will go back to a HEALTH_OK state.

I think that is a good thing, and certainly preferable to the current
nonsense.  If we want to distinguish between down+out OSDs that were failed
vs those that have been "acknowledged" by an admin to be dead, we will
need to add some additional state (possibly reusing the AUTOOUT flag?), but
that will require more discussion.

Backport: firefly (maybe)
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 55a97787088b79356c678a909b2410b3924e7f5b)
mon: fix typos, punctuation for mon disk space warning(s)

Backport: firefly
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 98883f6308ce72f69a71feab29ef00e13f319cdb)

Conflicts:

	src/mon/DataHealthService.cc
mon: refactor check_health()

Refactor the get_health() methods to always take both a summary and detail.
Eliminate the return value and pull that directly from the summary, as we
already do with the PaxosServices.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 82e47db8073b622183a5e33f6e0b999a3a144804)
mon: ensure HealthService warning(s) include a summary

The low disk space check would change our status to HEALTH_WARN and include
a detail message, but no summary.  We need both.

Backport: firefly
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 3ed7f2dd4345633ff34017b201082f4c261ef387)
Merge pull request #2057 from ceph/wip-8593-firefly

mon: backport health check improvements

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>