devstack: set ODL_GATE_SERVICE_PROVIDER to fullstack/functional tests
devstack: set ODL_GATE_SERVICE_PROVIDER to fullstack/functional tests

ODL_GATE_SERVICE_PROVIDER isn't set for fullstack/functional tests.
Set it to vpnservice for them.

Change-Id: I73cd2ec749bcc23f4a86b059c53f7daf32a8d4bf

openstack/networking-odl,openstack/networking-odl,openstack/networking-odl