Do not use skip_checks in TestServerAdvancedOps

if CONF.compute.flavor_ref_alt == CONF.compute.flavor_ref,
test_server_sequence_suspend_resume will be skipped unexpectedly,
so the skip judgement shouldn't be put in skip_checks.

Besides, if flavor_ref_alt is not set, the resize testcase should
also be skipped.

Change-Id: I6bc3680ffa8c440590e234ce17bb57f06b361d02
diff --git a/tempest/scenario/test_server_advanced_ops.py b/tempest/scenario/test_server_advanced_ops.py
index 4d9e59c..d6412eb 100644
--- a/tempest/scenario/test_server_advanced_ops.py
+++ b/tempest/scenario/test_server_advanced_ops.py
@@ -37,13 +37,6 @@
     """
 
     @classmethod
-    def skip_checks(cls):
-        super(TestServerAdvancedOps, cls).skip_checks()
-        if CONF.compute.flavor_ref_alt == CONF.compute.flavor_ref:
-            msg = "Skipping test - flavor_ref and flavor_ref_alt are identical"
-            raise cls.skipException(msg)
-
-    @classmethod
     def setup_credentials(cls):
         cls.set_network_resources()
         super(TestServerAdvancedOps, cls).setup_credentials()
@@ -51,6 +44,11 @@
     @decorators.idempotent_id('e6c28180-7454-4b59-b188-0257af08a63b')
     @testtools.skipUnless(CONF.compute_feature_enabled.resize,
                           'Resize is not available.')
+    @testtools.skipUnless(CONF.compute.flavor_ref !=
+                          CONF.compute.flavor_ref_alt
+                          and CONF.compute.flavor_ref_alt != "",
+                          'The flavor_ref_alt option should not be empty and '
+                          'should not be identical with flavor_ref')
     @test.services('compute', 'volume')
     def test_resize_volume_backed_server_confirm(self):
         # We create an instance for use in this test