Restructure the vpnaas roles

As neutron-vpnaas-agent has been loaded just inside of the existing l3 agent
rather than requiring operators to run a completely different binary with a
subclass of the existing L3 agent[1]. We need restructure this role to fit
with this new feature.

[1] https://review.openstack.org/488247

Depends-On: I47cd8ba5a14da3c76d5b1eb0b4c0cf0c729eb2ff
Change-Id: Id690a652bc9facf1c3e39358f548ab7ddd967d80
Implements: blueprint restructure-neutron-vpnaas
Closes-Bug: #1731498