Add ability to use gmetric instead of the embedded Ganglia library

... of reasons why you'd

want to do this.

1. Single configuration ie. if you already run gmond you already have gmond.conf
configured so no need to specify
2. Send to multiple destinations ie. gmetric allows you to specify multiple
udp_send_channels which can be unicast, multicast or both. We could implement this
using the embedded python library but it becomes messy