Fix coverage report generation

Coverage is a little bit confused by oslo.messaging as the pypi package
name. So we have to be explicit here or we get no data collection.

Adding a .coveragerc helps give the report more context as we don't
care about coverage in the tests or oslo-incubator.

Change-Id: I3d24187890efd7aecad7fcb7a2974b87e89ef6ff

.coveragerc

@@ -0,0 +1,7 @@
+[run]
+branch = True
+source = oslo_messaging
+omit = oslo_messaging/tests/*,oslo_messaging/openstack/*
+
+[report]
+ignore-errors = True

tox.ini

@@ -15,7 +15,7 @@ commands = flake8
 [testenv:cover]
 setenv = VIRTUAL_ENV={envdir}
 commands =
-  python setup.py testr --coverage
+  python setup.py test --coverage --coverage-package-name=oslo_messaging --testr-args='{posargs}'
 
 [testenv:venv]
 commands = {posargs}