Add VPNaaS service support

VPNaaS is a new service in Neutron (Havana release).
This patchs aims to support default driver (OpenSwan) by installing
packages & configuring vpn_agent.ini.

implement blueprint vpnaas-support
Change-Id: I7f8a3fbc1b175eb0ea3a8868a04ea648c2b8f1b2
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge "Add VPNaaS service support"