Update Ganglia to 3.5.7

And switch from hard-coded (and partly silly-named) paths to more
standard ones:
$ganglia_webdir = "/srv/org/wikimedia/ganglia-web-latest"
$ganglia_confdir = "/srv/org/wikimedia/ganglia-web-conf"

Change-Id: I0a88614504abc05fe0ec56d93b59f03db5281b34