Migrate ID generation for users/groups from controller to manager

The implementation of multi-backend unique IDs requires that the
manager layer is in charge of ID generation.  This patch lays that
ground work.  No functional change should be observed from this patch.

Partially Implements: blueprint multi-backend-uuids

Change-Id: I2253aa9872d1c9950ef35851bd3f11d7ab5e433b
Merge "Migrate ID generation for users/groups from controller to manager"