Move v3_to_v2_user from manager to controller

Managers should have no knowledge of v2 or v3, only the controllers
should, so this moves the v3_to_v2_user function from the identity
manager to common.controller.V2Controller.

Change-Id: I0c111245965a2578cfe1d7c9f1ca2df2f612b0d9
Merge "Move v3_to_v2_user from manager to controller"