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/api/network/test_subnetpools_extensions.py b/tempest/api/network/test_subnetpools_extensions.py
index be6bffc..01d7db2 100644
--- a/tempest/api/network/test_subnetpools_extensions.py
+++ b/tempest/api/network/test_subnetpools_extensions.py
@@ -46,7 +46,7 @@
             msg = "subnet_allocation extension not enabled."
             raise cls.skipException(msg)
 
-    @test.attr(type='smoke')
+    @decorators.attr(type='smoke')
     @decorators.idempotent_id('62595970-ab1c-4b7f-8fcc-fddfe55e9811')
     def test_create_list_show_update_delete_subnetpools(self):
         subnetpool_name = data_utils.rand_name('subnetpools')