elasticsearch: support for grouping stats for logstash indices

Change-Id: I5770fafa863655d289c74dfeb512d0f2cc5746d8
Merge branch 'es' of https://github.com/rtoma/Diamond into es

Conflicts:
	src/collectors/elasticsearch/elasticsearch.py
	src/collectors/elasticsearch/test/testelasticsearch.py

Change-Id: I8d88e3ce08fdf0cf261e340709f6c7b3f3cd00a0