The change in:
If1bc6151e8bb9b23eaac94662f4d18ff63040af2
accidentally switch the charm to use the neutron-vpn-agent for OVSi
ODL deployments; this is incorrect and breaks functional tests in
the odl-controller charm.
Moved the logic that switches l3->vpn back into OVS plugin mode
only so that OVS ODL uses l3-agent always.
Change-Id: I968a09217a5058191509650d080752d2a4aa1455