Move the `attr` decorator from test.py to tempest/lib

I think it's a good idea to move all utility decorators into
tempest/lib/decorators.py. This patch does that for the `attr`
decorator.

Change-Id: Iaafbb112b6eee458089cc49918359a8a8d0485e2
diff --git a/tempest/scenario/test_aggregates_basic_ops.py b/tempest/scenario/test_aggregates_basic_ops.py
index cefa119..8408a1e 100644
--- a/tempest/scenario/test_aggregates_basic_ops.py
+++ b/tempest/scenario/test_aggregates_basic_ops.py
@@ -96,7 +96,7 @@
         return aggregate
 
     @decorators.idempotent_id('cb2b4c4f-0c7c-4164-bdde-6285b302a081')
-    @test.attr(type='slow')
+    @decorators.attr(type='slow')
     @test.services('compute')
     def test_aggregate_basic_ops(self):
         self.useFixture(fixtures.LockFixture('availability_zone'))