Update 3PAR iSCSI Documentation to require 3.1.2 MU3

3PAR iSCSI Driver was updated and requires the 3PAR Firmware version
to be 3.1.2 MU3

Driver update review link: https://review.openstack.org/#/c/68456/

Change-Id: I3f82e553517bf21755afc23ff94b70144b6c45a9
Partial-Bug: #1269515
Merge "Update 3PAR iSCSI Documentation to require 3.1.2 MU3"