Move user fileds type check to identity.Manager

The fileds type's checking logic during creating and updating
users apply to all driver calls. It should be centralized in the
identity.Manager rather that continuing the trend of spreading them out
between controllers, managers and drivers.

This patch move the enable type checking logic to identity.Manager and
modify the related test cases in test_backend.py.

Change-Id: I37df56a61cd5ab332dcc9d74a7e99ee9041aa32e
Merge "Move user fileds type check to identity.Manager"