Replace the deprecated tempest.test.attr with decorators.attr

[1] moves the attr decorator from test.py to tempest/lib. So, all the
references to tempest.test has to be moved to tempest.lib.decorator.

[2] https://review.openstack.org/#/c/456236/

Change-Id: I17a6bb8728c9184dd7aeecf7f3b4cab62abd9680
diff --git a/ironic_tempest_plugin/tests/test_basic.py b/ironic_tempest_plugin/tests/test_basic.py
index dd14a40..bae615e 100644
--- a/ironic_tempest_plugin/tests/test_basic.py
+++ b/ironic_tempest_plugin/tests/test_basic.py
@@ -148,7 +148,7 @@
 class InspectorSmokeTest(manager.InspectorScenarioTest):
 
     @decorators.idempotent_id('a702d1f1-88e4-42ce-88ef-cba2d9e3312e')
-    @test.attr(type='smoke')
+    @decorators.attr(type='smoke')
     @test.services('object_storage')
     def test_baremetal_introspection(self):
         """This smoke test case follows this very basic set of operations: