Add pytest to the test image. (kubeflow#278)

* pytest can run unittests and emit a junit xml file.
* It also has good support for E2E tests that need to take command
  line arguments

Related to kubeflow#270

images/Pipfile

@@ -37,6 +37,7 @@ protobuf = "==3.5.1"
 pylint = "==1.8.2"
 python-dateutil = "==2.6.1"
 pytz = "==2017.3"
+pytest = "==4.1.1"
 pyyaml = "*"
 requests = "==2.18.4"
 requests-oauthlib = "==0.8.0"