Port used_limits & used_limits_for_admin into v2.1

The used_limits & used_limits_for_admin 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: I57199273dcaf20b0b6618dfb436a92f04b139f52
Merge "Port used_limits & used_limits_for_admin into v2.1"