Adding network api xml support

Network API tests with xml formatted request body were missing.
I added code to support this feature.
In the test cases "test_list_subnets" and "test_list_networks",
response code check was also missing.
I also added this check in the code.

Change-Id: I876c720b93e0d0cded96de0549edd6115fa1c655
diff --git a/tempest/api/network/test_routers.py b/tempest/api/network/test_routers.py
index df85682..4f687b0 100644
--- a/tempest/api/network/test_routers.py
+++ b/tempest/api/network/test_routers.py
@@ -21,6 +21,7 @@
 
 
 class RoutersTest(base.BaseNetworkTest):
+    _interface = 'json'
 
     @classmethod
     def setUpClass(cls):