Hardcode metadata ipv6 to be off

We hit issues with dhcp ha where the same metadata
ipv6 address attempts to be present on two hosts.
We simply turn that off for now.

Author: JohnGarbutt

neutron/agent/dhcp/agent.py

@@ -746,7 +746,7 @@ def enable_isolated_metadata_proxy(self, network):
                     self._metadata_routers[network.id] = (
                         router_ports[0].device_id)
 
-        if netutils.is_ipv6_enabled():
+        if netutils.is_ipv6_enabled() and False:
             try:
                 dhcp_ifaces = [
                     self.call_driver(