VPNaaS service name is wrong

In Ubuntu the VPNaaS service name and package are
neutron-plugin-vpn-agent. In Debian the VPNaaS service name and package
are neutron-vpn-agent.

Closes-Bug: #1285314
(cherry picked from commit 1e3ff014972fb0f18f8509d404df583ec8a11762)

Change-Id: Id3a0e2e939d2608802b3b2b39634e6c8bb3c32f5