Add support for the agent extension to NVP Plugin.

Of DHCP and L3 agents, NVP uses DHCP; This patch adds support
for the extension to this plugin.

This patch also moves some tests around, as a result of the
refactory done in https://review.openstack.org/#/c/35266/
Some code duplication is also removed.

Implements blueprint nvp-agent-scheduler-extension

Change-Id: I4ca1148e81fdb39bf6b58eef806536439e9f0c6f
Merge "Add support for the agent extension to NVP Plugin."