Make Neutron NVP plugin future-versions friendly

Make Neutron NVP plugin future-versions friendly

This patch enables the plugin to deal correctly with
newer versions of the NVP platform; this is done by
allowing the plugin to fall back to the closest known
version of the NVP platform. This is acceptable behavior
as L2/L3 operations implemented by the platform are most
certainly backward compatible.

This patch also improves test coverage for this part of the
code, which is critical to the correct behavior of the plugin.

Fixes bug #1217479

Change-Id: Ifcce47c6ee2bc2bc96ddfe9298d252700aceca3f