Work items:
* Updated Elasticsearch and InfluxDB v2 storage backends in order to support
grouping on time.
* Added a "dt_from_ts()" function to "cloudkitty.tzutils", along with unit
tests. This function converts an epoch timestamp to a timezone-aware datetime
object.
* Added unit tests for GET /v2/summary to validate grouping by time works as
expected.
* Updated the API documentation.
Change-Id: Ia01fced0bdb3a9b389a89d56f02029b9456781c2
Story: 2006730
Task: 37153
0d8a636755