Add instance_get_active_by_window_joined to InstanceList

This is needed to remove get_active_by_window from nova compute API and
allow us to make the simple_tenant_usage API use Flavor objects.

Related to blueprint compute-manager-objects

Change-Id: I93354db2775f08346f9b812f1f1e78a4050efd4a
Merge "Add instance_get_active_by_window_joined to InstanceList"