Add SQLAlchemy implementation of groupby

Implements: blueprint api-group-by

New class StatisticsGroupByTest contains the storage tests for group by
statistics and has its own test data

The storage tests check group by statistics for
 1) single field, "user-id"
 2) single field, "resource-id"
 3) single field, "project-id"
 4) single field, "source"
 5) single metadata field (not yet implemented)
 6) multiple fields
 7) multiple metadata fields (not yet implemented)
 8) multiple mixed fields, regular and metadata (not yet implemented)
 9) single field groupby with query filter
10) single metadata field groupby with query filter (not yet implemented)
11) multiple field group by with multiple query filters
12) multiple metadata field group by with multiple query filters
    (not yet implemented)
13) single field with period
14) single metadata field with period (not yet implemented)
15) single field with query filter and period
16) single metadata field with query filter and period (not yet implemented)

It also includes the implementation for the SQLAlchemy driver.

Change-Id: I902db657e424b9894c0382db4869b22317fc25da
Merge "Add SQLAlchemy implementation of groupby"