Port simple_tenant_usage into v2.1

The simple_tenant_usage extension is removed from v3 API. For now,
we need porting it back. And also try to share unittests between v2
and v2.1.

Partially implements blueprint v2-on-v3-api

Change-Id: Ie0c36fcd37fb188c1f1b57f04733bfa267b65201
Merge "Port simple_tenant_usage into v2.1"