varnish: add Ganglia metrics for memory statistics

Per discussion with Domas, to debug both the weird kswapd issues as well
as XFS memory allocation failures we've been seeing, add three Ganglia
Python plugins that monitor /proc/meminfo, /proc/vmstat and
/proc/buddyinfo.

These really belong to a base class to be applied in all hosts rather
than just Varnish, but let's try them first on a more limited basis with
the default settings before we apply everywhere.

The plugins were copied from
https://github.com/ganglia/gmond_python_modules/

Change-Id: I6a6d2cf1834a5a1bab9a243ceeb3dbacc9e60a58
Merge "varnish: add Ganglia metrics for memory statistics" into production