Allow `tox -epy27` to successfully run

Running `tox -epy27` currently without proper dependencies result in
an error.  See [1].  This fixes the tox.ini and the test-requirements.txt
so the py27 gate can correctly run.

[1] http://logs.openstack.org/74/438974/2/check/gate-kosmos-specs-python27-ubuntu-xenial/4da857f/console.html.gz

Change-Id: I3a79d2ad9e6acc3909e7650f52c870d31744dfb1

test-requirements.txt

@@ -0,0 +1,2 @@
+testrepository>=0.0.18
+testtools>=0.9.34

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 1.6
-envlist = docs
+envlist = docs,py27
 skipsdist = True
 
 [testenv]
@@ -10,6 +10,8 @@ setenv =
    VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
+commands =
+   python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:venv]
 commands = {posargs}