Pass public_address to creation of new cluster (dask#303)

Currently a cluster created by a gateway with a `public_address` different from `address` doesn't inherit it.

Author: zonca

dask-gateway/dask_gateway/client.py

@@ -635,6 +635,7 @@ def new_cluster(self, cluster_options=None, shutdown_on_close=True, **kwargs):
         return GatewayCluster(
             address=self.address,
             proxy_address=self.proxy_address,
+            public_address=self._public_address,
             auth=self.auth,
             asynchronous=self.asynchronous,
             loop=self.loop,