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>
mon: fix typos, punctuation for mon disk space warning(s)

Backport: firefly
Signed-off-by: Sage Weil <sage@inktank.com>
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>
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>
Merge pull request #1973 from ceph/wip-8593

mon: fix up health warnings

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>