Make 2 functions in network_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 add_router_interface,remove_router_interface of network_client use **kwargs.

Partially implements blueprint consistent-service-method-names

Change-Id: Ib6e222431c9681d01890bc980547cc2e90923526
diff --git a/tempest/services/network/resources.py b/tempest/services/network/resources.py
index 10911f7..0a7da92 100644
--- a/tempest/services/network/resources.py
+++ b/tempest/services/network/resources.py
@@ -89,14 +89,13 @@
 
     def add_to_router(self, router_id):
         self._router_ids.add(router_id)
-        self.network_client.add_router_interface_with_subnet_id(
-            router_id, subnet_id=self.id)
+        self.network_client.add_router_interface(router_id,
+                                                 subnet_id=self.id)
 
     def delete(self):
         for router_id in self._router_ids.copy():
-            self.network_client.remove_router_interface_with_subnet_id(
-                router_id,
-                subnet_id=self.id)
+            self.network_client.remove_router_interface(router_id,
+                                                        subnet_id=self.id)
             self._router_ids.remove(router_id)
         self.subnets_client.delete_subnet(self.id)