Add generic nose/testtools attr decorator

Adds support for a tempest specific attr decorator which calls
the nose.plugsin.attrib attr decorator as well as the
testtools.testcase attr decorator if it is type='smoke'.

This will allow us to more smoothly transition to testtools attr
support without losing any nose functionality and preserve use of
attributes such as positive/negative/whitebox etc indefinitely.

Change-Id: Ib1ca7b5ed313b7dda55e68357f21186368df0ce6
Merge "Add generic nose/testtools attr decorator"