Fix documentation around regions

It turns out region_name is an important parameter - it's not
just another kwarg that will get passed through.

Change-Id: I5cca8d324a1dcd1355991df793fe29eedfa15dc0

README.rst

@@ -247,7 +247,7 @@ Get a named cloud.
   import os_client_config
 
   cloud_config = os_client_config.OpenStackConfig().get_one_cloud(
-      'hp', 'region-b.geo-1')
+      'hp', region_name='region-b.geo-1')
   print(cloud_config.name, cloud_config.region, cloud_config.config)
 
 Or, get all of the clouds.

os_client_config/config.py

@@ -551,6 +551,7 @@ class OpenStackConfig(object):
             An argparse Namespace object; allows direct passing in of
             argparse options to be added to the cloud config.  Values
             of None and '' will be removed.
+        :param region_name: Name of the region of the cloud.
         :param kwargs: Additional configuration options
 
         :raises: keystoneauth1.exceptions.MissingRequiredOptions