Configure VPN when upgrading from Icehouse

There was a change in devstack how VPNaaS agent is started. It uses some
global variables introduced by https://review.openstack.org/#/c/86567/

Closes-Bug: #1313791
Change-Id: I76d8e8c9b9ac558f31cb3e22a936efbebff5b9a3
Merge "Configure VPN when upgrading from Icehouse"