Switch to decorators.idempotent_id on scenario

So many modules are using test.idempotent_id and this is for
switching to decorators.idempotent_id on scenario.

Change-Id: Ieae5a2ee6937ad4339aff531eadc5e4986d7ec42
Related-Bug: #1616913
diff --git a/tempest/scenario/test_snapshot_pattern.py b/tempest/scenario/test_snapshot_pattern.py
index 47c6e8d..374cdba 100644
--- a/tempest/scenario/test_snapshot_pattern.py
+++ b/tempest/scenario/test_snapshot_pattern.py
@@ -14,6 +14,7 @@
 #    under the License.
 
 from tempest import config
+from tempest.lib import decorators
 from tempest.scenario import manager
 from tempest import test
 
@@ -37,7 +38,7 @@
         if not CONF.compute_feature_enabled.snapshot:
             raise cls.skipException("Snapshotting is not available.")
 
-    @test.idempotent_id('608e604b-1d63-4a82-8e3e-91bc665c90b4')
+    @decorators.idempotent_id('608e604b-1d63-4a82-8e3e-91bc665c90b4')
     @test.services('compute', 'network', 'image')
     def test_snapshot_pattern(self):
         # prepare for booting an instance