Add support for globals_mute

[racavalier]

Could you add support in the gmond class for "globals_mute?" I'm setting up an intermediary collector running gmond that collects the output from a set of other nodes in a cluster and is polled by the Ganglia Meta server. I'd like for it to be muted, so it's not part of the monitoring, but can't set this in gmond.conf, since it's managed by the puppet module and that only allows mute to be set to the default of "no".