Run unit tests on py27 target

Doe to cover target inaccessibility,
let's use target for unit tests by default.
Also this will help on py3 later due to
some differences in logic in some helpers.

Change-Id: I33b3366953bcf14bd48c32c6cb1fdffe23242923

tox.ini

@@ -12,9 +12,15 @@ skip_missing_interpreters = True
 deps =
     -r{toxinidir}/fuelweb_test/requirements-devops-source.txt
     -r{toxinidir}/fuelweb_test/requirements.txt
+    mock>=1.2
+    pytest-cov
 usedevelop = False
 commands =
     ./run_system_test.py show-all-groups
+    # Run unit tests and coverage: groups collect does not cover API changes
+    py.test --cov-config .coveragerc --cov-report html --cov=core core/_tests
+    coverage html -d {envlogdir}
+    coverage report --fail-under 75
 
 [testenv:pep8]
 # TODO: #deps = hacking==0.7