Add a unit test coverage tox job

This commit adds a new tox job for running the tempest unit tests with
coverage enabled.

Change-Id: I36b5390ffd241e08e40be2d4b92b9e91a71cdc9e
diff --git a/test-requirements.txt b/test-requirements.txt
index 9486244..d7340f3 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -6,3 +6,4 @@
 oslo.sphinx
 mox>=0.5.3
 mock>=1.0
+coverage>=3.6