Include lswitch id in NSX plugin port mappings

This patch adds the logical switch identifier in the model class
for describing mappings between neutron and nsx switch ports.
This will allow for a better handling of the chained logical switch
use case.

This patch also moves the routines for retrieving nsx identifiers
in a separate module in preparation for the introduction of async
operations and renames relevant model classes, attributes, and
functions from 'nvp' to 'nsx'.

Related to blueprint nvp-async-backend-communication
Related to blueprint nicira-plugin-renaming

Change-Id: I5fa1e8f0d863b7d1fc278ffdeac8acc076a843e8
NVP: propagate net-gw update to backend

When a network gateway's name is updated, propagate the
update to the backend.

Closes-Bug: #1261334
Change-Id: Icaae84e7f5373946e8c82e0e519528409925dd9d
Merge "Include lswitch id in NSX plugin port mappings"
Merge "NVP: propagate net-gw update to backend"