Use veth instead of OVS internal ports by default.

[dceara]

We currently provision ports of type "internal" when simulating pods:
https://github.com/dceara/ovn-heater/blob/d5ae548dceae639cfb956149742a8fe557b2e393/ovn-tester/ovn_utils.py#L25
https://github.com/dceara/ovn-heater/blob/d5ae548dceae639cfb956149742a8fe557b2e393/ovn-tester/ovn_workload.py#L300
https://github.com/dceara/ovn-heater/blob/d5ae548dceae639cfb956149742a8fe557b2e393/ovn-tester/ovn_workload.py#L313

Using veth instead would make configuration look closer to what ovn-kubernetes provisions.

This should probably be configurable (to allow side-by-side tests).