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
Merge "Add support to PureISCSIDriver for Consistency Groups"