OpenStack Compute: Fix server model metadata

Updates the OpenStack compute server model so that it stores/handles
metadata correctly on Server create again.

This was a regression from 21499d8 (and should properly handle the
concern there too).

Includes an update to the test that fails without these fixes.
OpenStack: Add missing metadatum requests.

Add missing delete_meta and update_meta calls to the OpenStack
API. Also updates the model with some tests on these new calls.
Merge pull request #1597 from dprince/fix_openstack_metadata

OpenStack Compute: Fix server model metadata