Include lswitch id in NSX plugin port mappings

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