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
a2e42034f0