Add support for neutron nvp plugin

This patch adds support for NVP plugin configuration.

implement blueprint nvp-plugin-support
Change-Id: I8aff4fc6895ed5a3342e2e93d306014ec0982af8
(cherry picked from commit 25abac3c93442898203a42526cb4fb3d41fa140d)
Merge "Add support for neutron nvp plugin" into stable/havana