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: I24335128cb3566e4104ddc6dad9a53f1a5ec249b

.gitignore

@@ -14,7 +14,7 @@ dist
 
 .testrepository/
 .stestr/
-.coverage.*
+.coverage
 cover/
 
 *.sample

tox.ini

@@ -26,11 +26,14 @@ commands =
  stestr run {posargs}
 
 [testenv:cover]
+description = Calculates code coverage
+setenv =
+    PYTHON=coverage run --source monasca_notification --parallel-mode
 commands =
-  coverage erase
-  python setup.py test --coverage --testr-args='{posargs}' \
-    --coverage-package-name=monasca_notification
-  coverage report
+  stestr run {posargs}
+  coverage combine
+  coverage html -d cover
+  coverage xml -o cover/coverage.xml
 
 [testenv:debug]
 commands =