add the version requirement for testtools

when I run tempest, it display:
 'module' object has no attribute 'WithAttributes'.
But I have install testtools. when I upgrade testtools, the problem
is resovled. So I think we should require the version of testtools
in pip-requires.

Fixes bug 1133177

Change-Id: Ib30fa31f1a412981263bf93bc37c5f1d3918cf1f
Merge "add the version requirement for testtools"