Convert to use tempest attr implementation 1/2

Convert from nose attr decorator to use the tempest attr
decorator which calls both the nose and testtools attr
implementations.

Tests marked as smoke will be renamed when running under both
nosetests and testr but otherwise there is is no visible change. Eg.

tempest.tests.identity.admin.test_users.UsersTestXML.test_create_user

looks like:

tempest.tests.identity.admin.test_users.UsersTestXML.test_create_user[smoke]

Patch is broken into two completely separate parts to make it
easier to avoid merge conflicts with other changesets.
They do not need to be applied atomically and are not in any way dependent
on each other.

Change-Id: I310fced910820bacab07b6936f8b0a0a2a7990fa