Enforce compute:update policy in V3 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 V3 API level so that the compute:update policy will
continue to function as it did before.

This was fixed in I365ad8ac2845d914deefb841c314a50f4869dba0 for the V2
API.

Change-Id: I9caa3e10fe029569c6fd467e7b86103dd6590f9b
Closes-Bug: 1242955
Merge "Enforce compute:update policy in V3 API"