Add support for the extra route extension in the NVP plugin.

The underlying feature is available in NVP 3.2, which introduces a
new type of router. Therefore, create_lrouter needs to be made
version 'aware'.

This led to a number of fixes in the nvplib, especially around how
version is retrieved and how version-dependent methods are called.

Implements blueprint nvp-extra-route-extension

Change-Id: Ie4e2d93f70e1948a62563c8523aea61bb2194c84
Merge "Add support for the extra route extension in the NVP plugin."