* use StrongSwan driver instead of OpenSwan since xenial does not even provide
openswan packages anymore
* start and enable strongswan service (needs to be verified for centos)
* do not include ::l3_agent recipe in vpnaas recipe, since neutron-vpn-agent
fully replaces neutron-l3-agent
Change-Id: I81cd2e05273402e8db57f3ca5029fb4938bbfe29