Add ganglia monitoring for vhtcpd.

Monitor vhtcpd's internal statistics to help spot trends and possible issues
with cache purge requests. See [[bugzilla:43449#c14]].

- Gmond monitoring script that reads from /tmp/vhtcpd.stats
- vhtcpd.pyconf
- New puppet class varnish::monitoring::ganglia::vhtcpd
- Apply puppet class wherever varnish::htcppurger is used

Bug: 43449
Change-Id: I1f581a94dae5e16353060d3b49f5fe411854e77a