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

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