Replace test.attr with decorators.attr

Function 'tempest.test.attr()' has moved to 'tempest.lib.decorators
.attr()' in Pike and will be removed in a future version[1].
This patch replaces the 'tempest.test.attr()' with the 'tempest.lib
.decorators.attr().'

[1] Iaafbb112b6eee458089cc49918359a8a8d0485e2

Change-Id: If39f271124be63888073d3179090e508b4b1bc1f
1 file changed