Use ostestr as a tests runner

ostestr has more user-friendly output including lists of all and
failed tests.

Change-Id: I6db42b0f7480d79b85e20334fb3c2d8236efc316

test-requirements.txt

@@ -15,3 +15,4 @@ sphinx!=1.2.0,!=1.3b1,<1.3,>=1.1.2 # BSD
 tempest-lib>=0.14.0 # Apache-2.0
 testtools>=1.4.0 # MIT
 testrepository>=0.0.18 # Apache-2.0/BSD
+os-testr>=0.4.1 # Apache-2.0

tox.ini

@@ -13,7 +13,7 @@ passenv = *_proxy *_PROXY
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = find . -type f -name "*.pyc" -delete
-           python setup.py testr --testr-args='{posargs}'
+           ostestr {posargs}
 whitelist_externals = find
 
 [testenv:pep8]