Adds else and TODO in statistics storage tests

Fixes bug #1217688

Addresses minor issues that were raised in the code review comments
for Patch Set 10 of "Add SQLAlchemy implementation of groupby"
https://review.openstack.org/#/c/41597/

The issues were in the StatisticsGroupByTest class of the storage
driver tests.

1) Adds else clause to test_group_by_with_query_filter_multiple

   The else clause ensures that all possible values for 'project_id'
   and 'resource_id' are covered when checking the values returned
   by get_meter_statistics().

2) Adds TODO comments for tests that are not implemented

   The tests that have not been implemented are ones involving
   metadata fields, since group by with metadata fields has not yet
   been implemented.

Change-Id: I55cc0cb6cc791708c9b8038787d712760f9515df
Merge "Adds else and TODO in statistics storage tests"