Pass expected_attrs to instance_get_active_by_window_joined

This change sends the expected_attrs from the instance object
API down into the DB API so the client can filter on which
columns he actually needs to join when getting all instances
in a given time window.

The simple-tenant-usage API and _instance_usage_audit periodic
task are the only things calling this API and they already
provide specific expected_attrs so there is no risk of
regression due to the change. The DB API is backwards
compatible if columns_to_join is None, and expected_attrs
defaults to None already.

Change-Id: I3d8b5a1fd58783e465001bb5f10b5b87d69198d9
Partial-Bug: #1383469
Merge "Pass expected_attrs to instance_get_active_by_window_joined"