Use skip_checks if all testcases have same skip conditions

If all testcases in a testclass have same skip conditions,
we should use skip_checks, both to make the testcases skip as
early as possible and to make code clean.

Change-Id: I4173340e0d727e01ff3272ed2186f6f4c81ab24d
diff --git a/tempest/api/compute/security_groups/base.py b/tempest/api/compute/security_groups/base.py
index 5260a99..54a6da8 100644
--- a/tempest/api/compute/security_groups/base.py
+++ b/tempest/api/compute/security_groups/base.py
@@ -24,6 +24,12 @@
 class BaseSecurityGroupsTest(base.BaseV2ComputeTest):
 
     @classmethod
+    def skip_checks(cls):
+        super(BaseSecurityGroupsTest, cls).skip_checks()
+        if not utils.get_service_list()['network']:
+            raise cls.skipException("network service not enabled.")
+
+    @classmethod
     def setup_credentials(cls):
         # A network and a subnet will be created for these tests
         cls.set_network_resources(network=True, subnet=True)