consistency group updates for ibm storwize/svc driver

Implement the changes in the consistency_group handling as proposed in https://blueprints.launchpad.net/cinder/+spec/consistency-groups-kilo-update, in the ibm storwize/svc driver.

The changes are:
1. The ability to modify a consistecy group (update_consistencygroup)
2. The ability to create a consistency group from a consistency group snapshot or from another consistency group (create_consistencygroup_from_src)