ganglia_new: write a gmond.pid pidfile

varnish::instance compares the mtime of the gmond & Varnish pidfiles to
decide when to restart gmond. After the migration to ganglia_new & our
custom upstart script there's no pidfile, which makes gmond restart on
every puppet run.

Fortunately, gmond can write its own pidfile, so pass that option from
upstart and fix the constantly restarting ganglia-monitor process.

Change-Id: I91ddb2da440640386332bba8ae3d2b4b1ee54ac0