Make elasticsearch ganglia monitor compatible with logstash

The ganglia monitor for elasticsearch assumed that all clusters being
monitored would have search clients using stats groups to categorize
search traffic sources. This is not the case for the kibana frontend
being used with the logstash cluster. The lack of a 'groups' component
in the indices stats was causing the module to crash on initialization.

Bug: 61384
Change-Id: Ie56d3a5b55d628e783b09c7e1064e6d95fd63a48