Make update_extra_routes use **kwargs & doc string

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 update_extra_routes of network_client use **kwargs.
Also update doc string for the same.

Partially implements blueprint consistent-service-method-names

Change-Id: I4352ee72e4ccafd5a32cfb166bc692c5defe2c98
diff --git a/tempest/api/network/test_routers.py b/tempest/api/network/test_routers.py
index ed191b6..406ad44 100644
--- a/tempest/api/network/test_routers.py
+++ b/tempest/api/network/test_routers.py
@@ -301,7 +301,7 @@
 
         test_routes.sort(key=lambda x: x['destination'])
         extra_route = self.client.update_extra_routes(router['id'],
-                                                      test_routes)
+                                                      routes=test_routes)
         show_body = self.client.show_router(router['id'])
         # Assert the number of routes
         self.assertEqual(routes_num, len(extra_route['router']['routes']))
diff --git a/tempest/services/network/json/network_client.py b/tempest/services/network/json/network_client.py
index 08316be..b036520 100644
--- a/tempest/services/network/json/network_client.py
+++ b/tempest/services/network/json/network_client.py
@@ -307,13 +307,14 @@
                                                network_id)
         return self.delete_resource(uri)
 
-    def update_extra_routes(self, router_id, routes):
+    def update_extra_routes(self, router_id, **kwargs):
+        """Update Extra routes.
+
+        Available params: see http://developer.openstack.org/
+                              api-ref-networking-v2-ext.html#updateExtraRoutes
+        """
         uri = '/routers/%s' % router_id
-        put_body = {
-            'router': {
-                'routes': routes
-            }
-        }
+        put_body = {'router': kwargs}
         return self.update_resource(uri, put_body)
 
     def delete_extra_routes(self, router_id):