Add update to volume and snapshot controllers

Add update to volume and snapshot controllers

Add update method to api.openstack.volume.volumes and snapshots
controllers.  Url mapping automatically routes a PUT on the resource to
the update method.  Implementation closely matches updates to server
resource for the openstack compute api.

e.g.

curl http://localhost:8776/v1/${PROJECT_ID}/volumes/${VOLUME_ID} \
    -H 'x-auth-token: ${ADMIN_AUTH_TOKEN}' \
    -H 'content-type: application/json' \
    -XPUT -d '{"volume": {"display_name": "new-name"}}'

Add volume.api.API.update_snapshot method to avoid making db calls
directly from the SnapshotsController.

Add tests for new methods.

Update return value of test_volume.VolumeTestCase._create_snapshot
to return the whole model like _create_volume

fixes lp bug #1042904

Change-Id: I887178f9b7834cc7afd54bba8fb902b630faf8c0