[AIRFLOW-3295] Fix potential security issue in DaskExecutor (apache#4128

)

When user decides to use TLS/SSL encryption
for DaskExecutor communications,
`Distributed.Security` object will be created.

However, argument `require_encryption` is missed
to be set to `True` (its default value is `False`).

This may fail the TLS/SSL encryption setting-up.

airflow/executors/dask_executor.py

@@ -49,6 +49,7 @@ def start(self):
                 tls_client_key=self.tls_key,
                 tls_client_cert=self.tls_cert,
                 tls_ca_file=self.tls_ca,
+                require_encryption=True,
             )
         else:
             security = None