Trivial fix: Fix coverage tox.ini job

This is a trivial PS that fixes the tox -e cover job in
tox.ini which was recently broken with [0].

[0] https://review.gerrithub.io/#/c/405318/

Change-Id: Id50a6348e6f306c3d8d68fdd79eb331880e7498b

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py{35,27},py{35,27}-{postgresql},pep8,bandit,docs
+envlist = py{35,27},py{35,27}-{postgresql},cover,pep8,bandit,docs
 
 [testenv]
 usedevelop = True
@@ -52,10 +52,15 @@ commands =
   {toxinidir}/tools/functional-tests.sh '{posargs}'
 
 [testenv:cover]
+setenv = {[testenv]setenv}
+         PYTHON=coverage run --source deckhand --parallel-mode
 commands =
   coverage erase
   find . -type f -name "*.pyc" -delete
-  python setup.py testr --coverage --testr-args='{posargs}'
+  stestr run {posargs}
+  coverage combine
+  coverage html -d cover
+  coverage xml -o cover/coverage.xml
   coverage report
 
 [testenv:bandit]