Skip NetworkWritableMtuTest when driver is ML2/OVN

East/west icmp fragmentation is not supported with ML2/OVN since
there is no production use case, as mentioned in [1].
With this change NetworkWritableMtuTest will be skipped
automatically when ML2/OVN agents found and thus ci jobs
configs can be simplified by removing exclude list.

[1] https://docs.openstack.org/neutron/latest/ovn/gaps.html

Change-Id: I7f690b1a260ff70e12bcfac82f63e36ddca7cfbc
6 files changed