fixup: batching adapt etcd3gw client creation for Yoga

The api_path was only added to the client helper function in etcd3gw 2.1.0.
This is available in upper constraints for Zed.

Change-Id: Ide34499f64f8e8a92be80a132ece6090701733a9

Author: markgoddard

networking_generic_switch/batching.py

@@ -322,7 +322,7 @@ def __init__(self, switch_name, etcd_url=None, switch_queue=None):
             cert_key = params.get('cert_key')
             cert_cert = params.get('cert_cert')
             api_version = params.get('api_version', 'v3alpha')
-            etcd_client = etcd3gw.client(
+            etcd_client = etcd3gw.Etcd3Client(
                 host=host, port=port, protocol=protocol,
                 ca_cert=ca_cert, cert_key=cert_key, cert_cert=cert_cert,
                 api_path='/' + api_version + '/',