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
1 file changed