ansible: synchronize module needs '.docker_cmd' attr for Docker plugin.

ansible_mitogen/plugins/connection/mitogen_docker.py

@@ -42,3 +42,10 @@ import ansible_mitogen.connection
 
 class Connection(ansible_mitogen.connection.Connection):
     transport = 'docker'
+
+    @property
+    def docker_cmd(self):
+        """
+        Ansible 2.3 synchronize module wants to know how we run Docker.
+        """
+        return 'docker'