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/tox.ini b/tox.ini
index 6d596e3..88f2537 100644
--- a/tox.ini
+++ b/tox.ini
@@ -25,6 +25,10 @@
 setenv = OS_TEST_PATH=./tempest/tests
 commands = python setup.py test --slowest --testr-arg='tempest\.tests {posargs}'
 
+[testenv:cover]
+setenv = OS_TEST_PATH=./tempest/tests
+commands = python setup.py testr --coverage --testr-arg='tempest\.tests {posargs}'
+
 [testenv:all]
 setenv = VIRTUAL_ENV={envdir}
 commands =