mds: add volumes + status to data: section of 'ceph status'

mds: add volumes + status to data: section of 'ceph status'

This expands the data: section to include volume count and high-level
status, expanding the view of "logical" data services:

  data:
    volumes: 0/1 file systems healthy, 1 degraded
    pools:   3 pools, 65 pgs
    objects: 22 objects, 2.3 KiB
    usage:   1.0 GiB used, 100 GiB / 101 GiB avail
    pgs:     65 active+clean

or

  data:
    volumes: 1/1 file systems healthy
    pools:   3 pools, 65 pgs
    objects: 22 objects, 2.3 KiB
    usage:   1.0 GiB used, 100 GiB / 101 GiB avail
    pgs:     65 active+clean

Signed-off-by: Sage Weil <sage@newdream.net>