Enable Neutron VPN as Service

Add a recipe, related attribute/template, and unit tests to
to install, configure and start VPN service.
Stop L3 agent if VPN is enabled.

Change-Id: I63322e3c43e38444163371636cbab2c3bbdeaacf
Implements: Blueprint neutron-vpnaas-enablement
Merge "Enable Neutron VPN as Service"