Implement retype in SolidFire driver

Implement retype in SolidFire driver

This just adds the ability to modify the
type of an existing SolidFire volume to
another type suppported on the same backend.

Main use case for this is changing QoS settings
for a volume on-demand.

Also updates the internal attributes to store the
deltas between the old-type and new-type.

Change-Id: I65a4c34cbec17d0f3987c1d964e6ad1bafea3601
Implements: blueprint retype-support-for-solidfire-driver