Fix coverage job configuration

Set up coverage job configuration as recommended in Project Testing
Interface documentation.

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

Change-Id: I6db38500945a96fffab452c5be5b0ef56ff1f6b9

tox.ini

@@ -29,11 +29,13 @@ commands =
   ostestr {posargs}
 
 [testenv:cover]
+setenv =
+    PYTHON=coverage run --source monasca_persister --parallel-mode
 commands =
-  coverage erase
-  python setup.py test --coverage --testr-args='{posargs}' \
-    --coverage-package-name=monasca_persister
-  coverage report
+    stestr run '{posargs}'
+    coverage combine
+    coverage html -d cover
+    coverage xml -o cover/coverage.xml
 
 [testenv:debug]
 commands = oslo_debug_helper -t monasca_persister/tests {posargs}