Fix and unify cover environment

Fix cover environment and unify it accordingly to Project Testing
Interface [1].

[1] https://governance.openstack.org/tc/reference/pti/python.html#coverage-jobs

Change-Id: I045d46d74809e506bcd94deaefc5477461616071

tox.ini

@@ -25,11 +25,14 @@ commands =
   stestr run {posargs}
 
 [testenv:cover]
+description = Calculates code coverage
+setenv =
+    PYTHON=coverage run --source monasca_api --parallel-mode
 commands =
-  coverage erase
-  python setup.py test --coverage --testr-args='{posargs}' \
-    --coverage-package-name=monasca_api --omit=monasca_api/hacking/*
-  coverage report
+  stestr run {posargs}
+  coverage combine
+  coverage html -d cover
+  coverage xml -o cover/coverage.xml
 
 [testenv:debug]
 commands =