Pass the correct arguments to check_ganglia.

Our queries to ganglia were missing the 'cluster' parameter that
check_ganglia uses to query gmetad, resulting in us getting back from
gmetad all data for all metrics, and not only the ones for the desired
cluster/host combination as check_ganglia was passing 'None' as the
cluster name.

For example:

$ echo '/Miscellaneous eqiad/fluorine' | nc nickel.wikimedia.org 8654 \
     | wc -c
1066388

$ echo '/None/fluorine' | nc nickel.wikimedia.org 8654 | wc -c
23567370

This should reduce a lot the stress on memory on neon.

Change-Id: I72fa213f41ecff0d3b6a8a6274c76ecdcff53953
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>