Remaining OF 1.3 rules after full neutron resources removal

(Using Helium release and VxLAN overlays), some OpenFlows rules remain after deleting all the neutron resources. It fails (at least) for ports which device_owner is equal to network:dhcp.

This bug was introduced by Helium release.

To reproduce it (ODL MD in master):

• create a private network and a subnet (DHCP activated)
• launch a VM
• delete the VM, next the subnet, next the network.

In my platform, the DHCP servers and the VMs are running on different servers.

The patch "make delete_router send delete_port to core_plugin" [1] could be applied to ODL MD to check OF 1.3 rules for gateway ports.

[1] https://review.openstack.org/#/c/116924/