[dask] Add scheduler address to dask config. (#7581)

- Add user configuration.
- Bring back to the logic of using scheduler address from dask.  This was removed when we were trying to support GKE, now we bring it back and let xgboost try it if direct guess or host IP from user config failed.

tests/python/test_tracker.py

@@ -28,7 +28,7 @@ def run_rabit_ops(client, n_workers):
     from xgboost import rabit
 
     workers = _get_client_workers(client)
-    rabit_args = client.sync(_get_rabit_args, len(workers), client)
+    rabit_args = client.sync(_get_rabit_args, len(workers), None, client)
     assert not rabit.is_distributed()
     n_workers_from_dask = len(workers)
     assert n_workers == n_workers_from_dask