updated ApiClient of python sdk.

When create api instance, if we don't pass the `api_client` parameter,
then use the default api_client in `config` module.

modules/swagger-codegen/src/main/resources/python/config.mustache

@@ -0,0 +1,8 @@
+from __future__ import absolute_import
+
+from .swagger import ApiClient
+
+# Configuration variables
+
+api_client = ApiClient("{{basePath}}")
+