Adapt VPN agent to use new main for L3 Agent
Adapt VPN agent to use new main for L3 Agent

Patch [1] broke out the main logic out of the L3 agent module, but
it kept the old entry point for bw compat. This patch follows up to
reflect the new location.

[1] https://review.openstack.org/#/c/145979/

Change-Id: Ie624f66e7977b57bb83b4bad93767d592f482790
Partially-Implements: bp restructure-l3-agent

openstack/neutron-vpnaas,openstack/neutron-vpnaas