3PAR iSCSI volumes attach to single nsp

Update the 3PAR iSCSI Driver to correctly identify
the least used iSCSI Node:Slot:Port (NSP) and only attach
the volume using that single NSP.

DocImpact: Driver now requires 3PAR firmware version 3.1.2 MU3
Ref: https://review.openstack.org/#/c/68513/

Change-Id: I1627e44a5168a7dbd19ea62312d65a00044753e9
Closes-Bug: #1269515
Convert ceph backup unit tests from mox to mock

Closes-Bug: 1261866
Change-Id: Ia1e5e971c505a4f5cc00898c6a946d5dee45c29b
Merge "3PAR iSCSI volumes attach to single nsp"