mon/PGMonitor: calc the %USED of pool using used/(used+avail)

mon/PGMonitor: calc the %USED of pool using used/(used+avail)

we were using
"the raw space used by this pool" / "the raw space of the whole cluster"
as the %USED. but it's wrong:

- there is chance that not all OSDs are assigned to the pool in question
- we can not utilize all the raw space for the pool: there is overhead.
  and the size of available space for a pool is capped by the assigned
  OSD with minimal free space.

so we should use
 USED / (USED + AVAIL)
as the %USED. so once we have no AVAIL space left, %USED will be 100%.

Fixes: http://tracker.ceph.com/issues/16933
Signed-off-by: Kefu Chai <kchai@redhat.com>