selftests: add openvswitch selftest suite

Previous commit resolves a WARN splat that can be difficult to reproduce,
but with the ovs-dpctl.py utility, it can be trivial.  Introduce a test
case which creates a DP, and then downgrades the feature set.  This will
include a utility 'ovs-dpctl.py' that can be extended to do additional
tests and diagnostics.

Signed-off-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>