Remove GET ops from update_router()

Each service client method should be a single REST operation, but
update_router() fetches original router information before PUT
operation. This kind of thing should be executed on test side.
This patch removes the GET operation from the method.

Partially implements blueprint consistent-service-method-names

Change-Id: Ie6ebb5f8e336e702b746ba9361fe452b390ffd8c
diff --git a/tempest/cmd/javelin.py b/tempest/cmd/javelin.py
index 17ee618..f2a69c7 100755
--- a/tempest/cmd/javelin.py
+++ b/tempest/cmd/javelin.py
@@ -823,7 +823,7 @@
         if router['gateway']:
             if CONF.network.public_network_id:
                 ext_net = CONF.network.public_network_id
-                client.routers._update_router(
+                client.routers.update_router(
                     router_id, set_enable_snat=True,
                     external_gateway_info={"network_id": ext_net})
             else: