refactor core code to nexus automated vPC

[Carol Bouchard]

As part of my code review for automated vPC, there was a method in the RESTAPI driver which was difficult to read. This code set aims to clean that up.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/485801

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/485801
Committed: https://git.openstack.org/cgit/openstack/networking-cisco/commit/?id=e4b2af943a89bce8e80d02fd0f43ca1232c0128c
Submitter: Jenkins
Branch: master

commit e4b2af943a89bce8e80d02fd0f43ca1232c0128c
Author: Carol Bouchard <email address hidden>
Date: Thu Jul 20 15:59:17 2017 -0400

    Refactor/Refine the core automated vPC code

    The main objective of this changeset is to refactor and refine the
    large method which is core to Nexus automated port channel creation.
    While refactoring, I found that we can also generate the action
    'switchport mode trunk' without regard to whether it is a
    port channel interface or ethernet. Also added to this changeset are
    failure test cases for the refactored/refined method.

    Change-Id: I4125b33375c1b5a956041ef04db294f2024b6520
    Closes-bug: #1705294