Constants were in
neutron/plugins/ml2/drivers/openvswitch/agent/common/constants.py
but at least some of them, like TUNNEL_NETWORK_TYPES,
PEER_INTEGRATION_PREFIX or FLOOD_TO_TUN are used in other projects also
so it would be good to move have them in the neutron-lib.
Change-Id: I46f0cc0bbf680fbceff1806720bb40b60a5e54e8