Introduce iSCSI driver for Pure Storage FlashArray

This patch introduces an iSCSI driver for Pure Storage FlashArray to Cinder.
It impliments all required features.

Certification test results:
    https://bugs.launchpad.net/cinder/+bug/1347109

Change-Id: I25f465b4abfc19d9b2717ec095c65201e76beab9
Impliements: blueprint pure-iscsi-volume-driver
Merge "Introduce iSCSI driver for Pure Storage FlashArray"