Update metrics.py, esquery.py and test_esquery.py to use only elasticsearch_dsl module

[aswanipranjal]

After #57, this issue adds the use of elasticsearch_dsl to other files such as metrics.py.

esquery.py in the function get_aggs, returns a json dict which is then used in metrics.py to get the results via requests.get method.
This issue is about removing completely the direct querying to elasticsearch and using elasticsearch_dsl.py instead.

EDIT: The second PR is ready, Please review #58 so that the second PR can be added on top of it.