Add policy checking to compute.api.get_all_instance_metadata

Retrieves all the instance metadata and the list of instances, and
then checks the policy for each metadata against the matching instance.

Fixes bug #1192715

Change-Id: I5c4542250c784a868c6cb16a21823eab7b98fc63
Progress towards DB approach on 1192715

Change-Id: I27c1d0a4f6d8f027847276925f0c16f01ede7491
Fix test_instance_metadata_get_all test

Change-Id: I32ae327316580f2481ac0bbfdcc4ad4269247ac7