Add vpnaas_agent_package parameter to FWaaS

On debian platforms, when using both VPNaaS and FWaaS, puppet will
remove neutron-vpn-agent and install neutron-l3-agent instead.
This behavior brokes VPNaaS.

As neutron-vpn-agent package is a superset of neutron-l3-agent, we
should install neutron-vpn-agent instead of neutron-l3-agent when
needed.

This patch will add a boolean parameter called vpnaas_agent_package
to neutron::services::fwaas in order to give the choice to install
neutron-vpn-agent package instead of neutron-l3-agent package on
debian platforms.

Change-Id: I812dc1b53c77fe2e01ccd8a4e3f718c9a990f77c
Closes-Bug: 1328534