CephCollector: add options for disabling stats

For larger systems, the stat collection can be
overly verbose and cause excessive load on low
powered graphite servers.  We introduce 'osd_stats_enabled'
and 'long_running_detail' to provide a flexible way
to tone down the verbosity.
Merge pull request #2 from ceph/wip-7600

CephCollector: add options for disabling stats