Adds DVR related env variable to devstack-vm-gate.sh

This is going to be used when setting up Jenkins job for
Neutron DVR.

Supports-blueprint: neutron-ovs-dvr

Change-Id: Iefa4a4826ba01d5011ef1b0bb04190ebc96b6ea6
Merge "Adds DVR related env variable to devstack-vm-gate.sh"