mon: make 'ceph -s' show pg state counts in reverse descending order

Because these are in an unordered_map we print this is random order.
Instead sort descending by count.

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #3231 from ceph/wip-mon-health

mon: make 'ceph -s' show pg state counts in reverse descending order

Reviewed-by: Loic Dachary <ldachary@redhat.com>