Make volumes_extensions_client use **kwargs

As we discussed on
http://lists.openstack.org/pipermail/openstack-dev/2015-July/068864.html
All http POST/PUT methods need to contain **kwargs as their arguments.
This patch makes volumes_extensions_client use **kwargs.

Partially implements blueprint consistent-service-method-names

Change-Id: Ie7f61741d54a629a15b1e2b8fce178ebc06176d3
diff --git a/tempest/clients.py b/tempest/clients.py
index 7cb4347..b9f7991 100644
--- a/tempest/clients.py
+++ b/tempest/clients.py
@@ -321,8 +321,7 @@
             'build_timeout': CONF.volume.build_timeout
         })
         self.volumes_extensions_client = VolumesExtensionsClient(
-            self.auth_provider, default_volume_size=CONF.volume.volume_size,
-            **params_volume)
+            self.auth_provider, **params_volume)
 
     def _set_database_clients(self):
         self.database_flavors_client = DatabaseFlavorsClient(