Adds unit test coverage

Adds the coverage and stestr modules to tox so
that unit test code coverage reports can be
generated.

Change-Id: If67b31be5996de95e60ad73f63124e181ec955b5
Signed-off-by: beierl <mbeierl@vmware.com>
diff --git a/test-requirements.txt b/test-requirements.txt
index c67a4b1..5741c91 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -22,3 +22,5 @@
 flake8<3.0
 mock
 requests-mock
+coverage==4.5.3
+stestr>=1.0.0