Make Ceph collector to pick up cluster name

Previously the prefix on admin socket names
was a config setting, had to be manually set
for one and only one cluster name.  In the
output stats, the cluster name was not used,
'ceph' was always passed through.

This change makes the collector gather stats
for all clusters it sees, and include the
name of the cluster in the metric names.
Extend Ceph collector to collect pool stats

These stats are only present in more recent versions
of Ceph (currently >=0.72, but expected to be backported).

Because these stats are for cluster-wide pools rather than
an individual host/service, they are reported with a global
name, ceph.<cluster name>.pool.<pool id>.*
CephCollector: Gather 'ceph df' statistics
Separate cluster-wide from service stats, and
read the 'ceph pg dump summary' output to
get a cluster-wide OP/s figure