Switch on coverage support for unit tests

After we merge this patch we can use 'tox -e cover' and see how is our
code is covered with tests. It's very good for proper code process.

Change-Id: Ia39145727b0dc55879cd807e2ef4fc2de331c9a4
Implements: blueprint coverage-for-testr

test-requirements.txt

@@ -11,3 +11,4 @@ discover
 fixtures>=0.3.14
 testrepository>=0.0.17
 testtools>=0.9.32
+coverage>=3.6

tox.ini

@@ -19,7 +19,10 @@ commands =
 sitepackages = False
 
 [testenv:cover]
-commands = nosetests --no-path-adjustment --with-coverage --cover-erase --cover-package=climate --cover-inclusive []
+setenv = VIRTUAL_ENV={envdir}
+commands =
+  python setup.py testr --coverage \
+    --testr-args='^(?!.*test.*coverage).*$'
 
 [testenv:pep8]
 commands = flake8 {posargs}