Fix the removal of skip_unless_config decorator in Tempest

That decorator was not really useful as there's already a similar
decorator in testtools.

Note, I am still going to propose something (worst case a revert) to
fix the already released version of this Tempest plugin.

Change-Id: I578c5b5e1c83aa7a41a334a49eceff2b90a60767
diff --git a/requirements.txt b/requirements.txt
index 9d93007..80b424b 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -5,4 +5,5 @@
 dnspython>=1.14.0;python_version=='2.7' # http://www.dnspython.org/LICENSE
 dnspython3!=1.13.0,!=1.14.0,>=1.12.0;python_version>='3.0' # http://www.dnspython.org/LICENSE
 ddt>=1.0.1 # MIT
+testtools>=1.4.0 # MIT
 tempest>=14.0.0 # Apache-2.0