Fix test coverage tox command for patrole.

Currently, tox -ecover doesn't work. This patch simply uses
nosetests and nosecover to generate a correct coverage report for
unit tests.

Change-Id: Ib7093796ead2bbef5c588c304336582f06ddd1c3
diff --git a/tox.ini b/tox.ini
index ba00222..b4953e7 100644
--- a/tox.ini
+++ b/tox.ini
@@ -28,7 +28,15 @@
 commands = {posargs}
 
 [testenv:cover]
-commands = python setup.py test --coverage --testr-args='{posargs}'
+setenv = VIRTUAL_ENV={envdir}
+         NOSE_WITH_COVERAGE=1
+         NOSE_COVER_BRANCHES=1
+         NOSE_COVER_PACKAGE=patrole_tempest_plugin
+         NOSE_COVER_HTML=1
+         NOSE_COVER_HTML_DIR={toxinidir}/cover
+         NOSE_WHERE=patrole_tempest_plugin/tests/unit
+whitelist_externals = nosetests
+commands = nosetests {posargs}
 
 [testenv:docs]
 commands = python setup.py build_sphinx