Add support for retype in Storwize/SVC driver

Add support for retype in Storwize/SVC driver

In Storwize/SVC, retype can be handled by a few cases:
1. The change may just require setting an option on the volume
2. The change may require moving the data in the same pool (e.g.,
   compression)
3. The scheduler may have chosen a different pool virtualized by the
   same Storwize or SVC, in which case we move the data as in #2

Implements: blueprint storwize-svc-retype

Change-Id: Ic2ed17bce56b5ea65e1596acfa85720cf4818fa8