Add support to PureISCSIDriver for Consistency Groups

Add support to PureISCSIDriver for Consistency Groups

This change adds implementations for the required driver methods for
Consistency Groups to be supported with the PureISCSIDriver. There is a
nice direct mapping between Consistency Groups and Purity Protection
Groups which makes the implementation pretty straightforward.

Implements: blueprint pure-iscsi-consistency-group
Change-Id: I8b97947d4788a7a7ba2dfa1d2b07645eafba76ec