Add missing correlation type "mpls" to Port Pair create
command's help (PortPairCreate).
The merged Port Pair command was still using the older help
description, before MPLS correlation support was merged to
networking-sfc.
Also add unit test coverage for correlation type "mpls".
Closes-Bug: #1708968
Change-Id: I708345b25b600ca94504725f1559988ae29413b8
(cherry picked from commit ff86783fd4)