Move skip condition under skip_checks method

Currently, the validation of existing multiple backends is preformed
in resource_setup stage in stead of skip_checks stage.

Change-Id: I4101ddc8b795276cc2327b2e38d7803d63913043
diff --git a/tempest/api/volume/admin/test_multi_backend.py b/tempest/api/volume/admin/test_multi_backend.py
index c0891e4..c5c70d2 100644
--- a/tempest/api/volume/admin/test_multi_backend.py
+++ b/tempest/api/volume/admin/test_multi_backend.py
@@ -29,6 +29,10 @@
         if not CONF.volume_feature_enabled.multi_backend:
             raise cls.skipException("Cinder multi-backend feature disabled")
 
+        if len(set(CONF.volume.backend_names)) < 2:
+            raise cls.skipException("Requires at least two different "
+                                    "backend names")
+
     @classmethod
     def resource_setup(cls):
         super(VolumeMultiBackendTest, cls).resource_setup()
@@ -41,9 +45,6 @@
 
         # Volume/Type creation (uses volume_backend_name)
         # It is not allowed to create the same backend name twice
-        if len(backend_names) < 2:
-            raise cls.skipException("Requires at least two different "
-                                    "backend names")
         for backend_name in backend_names:
             # Volume/Type creation (uses backend_name)
             cls._create_type_and_volume(backend_name, False)