Update test.attr to decorators.attr

Tempest has moved their attr decorator to tempest.lib [0]. This
change updates all test.attr decorators to the new location,
removing deprecation warnings.

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

Closes-Bug: #1683952
Change-Id: Ia0d47ab60f57ae0e1eee65527297f14afce378e0
8 files changed