Fix for $critical param on monitor_ganglia define

monitor_ganglia is a wrapper for monitor_service, and passes many
parameters straight through to monitor_service.  The $critical
parameter had been defined twice and used for different things.

This renames the $critical param passed to monitor_service to
$nagios_critical, which is the same name that monitor_graphite_threshold
uses.

Change-Id: I9f4bc069324d7baf930b8d34b3c1fe824b2ef74a