ceph: update the Ceph collector

1. The library routine subprocess.check_output was added in 2.7. This
patch implements a similar function on top of Popen in order to support
2.6.

2. Examines the performance counter schemas to make a better guess about
the data types being collected (e.g. counters, bytes, etc...)

3. Avoids creating deeper metric paths "osd.N", and renames these to be
osd-N which Graphite will not treat as a nested name.

4. Records the average since the last invocation of the collector for
the long-running average Ceph metric types.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>