Extend API of 'volume' app with endpoints for handling metadata of
volumes and snapshots:

Volumes:

* GET /volumes/<vol_id>/metadata: Show volume's metadata
* POST /volumes/<vol_id>/metadata: Update volume's metadata
* PUT /volumes/<vol_id>/metadata: Replace volume's metadata
* DELETE /volumes/<vol_id>/metadata/<meta_key>: Delete volume's metadata
                                                item

Snapshots:

* GET /snapshots/<vol_id>/metadata: Show snapshot's metadata
* POST /snapshots/<vol_id>/metadata: Update snapshot's metadata
* PUT /snapshots/<vol_id>/metadata: Replace snapshot's metadata
* DELETE /snapshots/<vol_id>/metadata/<meta_key>: Delete snapshot's metadata
                                                  item