Prefix Ganglia metrics with "ganglia_".

The gmond (Ganglia) exporter module exports many metrics not under our
control. They should all be prefixed in a common way to make it obvious
where they came from.

Fixes https://github.com/prometheus/node_exporter/issues/8
Merge pull request #9 from prometheus/prefix-ganglia-metrics

Prefix Ganglia metrics with "ganglia_".