dsvm tempest setup: use ovsfw
dsvm tempest setup: use ovsfw

Change I8cef6fc49b95b8a18fcb476da079fbc55fecd802 for bagpipe driver
improves coexistence between BGPVPN and a Neutron router, but requires
using the openvswitch firewall driver for single nodes setups.

Change-Id: I9d63ce7343f0eacc0bdc7eda568cc04586bdb4f8

openstack/networking-bgpvpn,openstack/networking-bgpvpn,openstack/networking-bgpvpn