Split resource_setup for network tests

Split up the resource_setup method for all the network tests, as per the
latest spec.

Partially-implements bp:resource-cleanup

Change-Id: If96a154cfe65dd3d18ff6b1e1943ccfb1db7300e
diff --git a/tempest/api/network/test_vpnaas_extensions.py b/tempest/api/network/test_vpnaas_extensions.py
index ba30326..4ab69e0 100644
--- a/tempest/api/network/test_vpnaas_extensions.py
+++ b/tempest/api/network/test_vpnaas_extensions.py
@@ -24,7 +24,6 @@
 
 
 class VPNaaSTestJSON(base.BaseAdminNetworkTest):
-
     """
     Tests the following operations in the Neutron API using the REST client for
     Neutron:
@@ -34,10 +33,14 @@
     """
 
     @classmethod
-    def resource_setup(cls):
+    def skip_checks(cls):
+        super(VPNaaSTestJSON, cls).skip_checks()
         if not test.is_extension_enabled('vpnaas', 'network'):
             msg = "vpnaas extension not enabled."
             raise cls.skipException(msg)
+
+    @classmethod
+    def resource_setup(cls):
         super(VPNaaSTestJSON, cls).resource_setup()
         cls.ext_net_id = CONF.network.public_network_id
         cls.network = cls.create_network()