create_cgsnapshot and create_consistencygroup_from_src can not update metadata of volume/snapshot

[jiamin]

Make create_consistencygroup_from_src as an example, volumes metadata returned by the driver will be not updated into DB by create_consistencygroup_from_src, because it only calls the volume_update to update the volume, but no volume_metadata_update.

if volumes_model_update:
    for update in volumes_model_update:
        self.db.volume_update(context, update['id'], update)

some users think that it is helpful to show the volume metadata, such as volume ID or data type of the volume. so it would be better to support it.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/346358

[OpenStack Infra (hudson-openstack)]

Change abandoned by jiamin (<email address hidden>) on branch: master
Review: https://review.openstack.org/346358