Convert project to os-testr

I am not sure how folks feel about this, but I think os-testr
makes failures a little easier to debug when tests fail.  For
one thing you get stdout/stderr in the test output.  Running a
single test is a little different:

    tox -e py27 -- --regex keystoneauth1.tests.unit.identity.test_token

Change-Id: Iea05633af3708c5e9443d86c8ddf947e68c0bebe

test-requirements.txt

@@ -14,6 +14,7 @@ oauthlib>=0.6
 oslo.config>=2.3.0 # Apache-2.0
 oslosphinx>=2.5.0 # Apache-2.0
 oslotest>=1.10.0 # Apache-2.0
+os-testr>=0.1.0
 pycrypto>=2.6
 requests-mock>=0.6.0 # Apache-2.0
 sphinx!=1.2.0,!=1.3b1,<1.3,>=1.1.2

tox.ini

@@ -12,7 +12,7 @@ setenv = VIRTUAL_ENV={envdir}
 
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-commands = python setup.py testr --testr-args='{posargs}'
+commands = ostestr {posargs}
 
 [testenv:pep8]
 commands =