Enforce compute:update policy in API

Now that compute/api.py:update() is no longer being called for instance
updates the policy check is no longer enforced.  This adds policy
checking back in at the API level so that the compute:update policy will
continue to function as it did before.

Bug 1212807

Change-Id: I365ad8ac2845d914deefb841c314a50f4869dba0
Merge "Enforce compute:update policy in API"