osds: use pg stat command instead of ceph status

osds: use pg stat command instead of ceph status

The ceph status command returns a lot of information stored in variables
and/or facts which could consume resources for nothing.
When checking the pgs state, we're using the pgmap structure in the ceph
status output.
To optimize this, we could use the ceph pg stat command which contains
the same needed information.
This command returns less information (only about pgs) and is slightly
faster than the ceph status command.

$ ceph status -f json | wc -c
2000
$ ceph pg stat -f json | wc -c
240
$ time ceph status -f json > /dev/null

real	0m0.529s
user	0m0.503s
sys	0m0.024s
$ time ceph pg stat -f json > /dev/null

real	0m0.426s
user	0m0.409s
sys	0m0.016s

The data returned by the ceph status is even bigger when using the
nautilus release.

$ ceph status -f json | wc -c
35005
$ ceph pg stat -f json | wc -c
240

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>