Update Ganglia metric module

- Change default prefix to 'eventlogging_' (was 'zeromq_', which implied that
  the module was still generic enough to be used to track any ZeroMQ publisher.
  It isn't.)
- Changed metric group parameter 'collect_every' to 30 (from 10). Produces
  smoother graphs.

Change-Id: I017a9931f3350ed19e6bbaa6e251a5c34c6ca368