half implemented munin server charm (for future use)

This is untested, and probably won't install. It's in a charm because
1. I'm not sure where else to put it
2. We're waiting for Juju to allow multiple services on one machine

There's actually a preexisting munin charm that we could've used, but
it's only in precise and quantal charm repos...
add munin
add MUNIN_SERVER to custard config