Make Ganglia metric module configuration-aware

The cost of implementing the metric module as a generic ZeroMQ publisher
monitor is that the streams have to be enumerated in the metric's pyconf file,
and Puppet sucks at assembling a file on the basis of multiple resources.
This patch refactors the metric module to discover streams via EventLogging
instance configuration files in <eventlogging.d/>.

Change-Id: I93504edbee39ca0640fc01072d1abc686521cd4d