Refining test coverage

This patch excludes the test artifacts from coverage results.
It uses coverage lib, already included in test-requirements,
to put the coverage results in 'cover' folder and displays
the report as well as part of tox `cover` target.

Change-Id: Ib6dd8558dc5db4025b27c660690d0c31fd983367

.coveragerc

@@ -4,3 +4,6 @@ source = sushy
 
 [report]
 ignore_errors = True
+
+[html]
+directory = cover

tox.ini

@@ -19,7 +19,13 @@ commands = flake8 {posargs}
 commands = {posargs}
 
 [testenv:cover]
-commands = python setup.py test --coverage --testr-args='{posargs}'
+# After running this target, visit sushy/cover/index.html
+# in your browser, to see a nicer presentation report with annotated
+# HTML listings detailing missed lines.
+commands = coverage erase
+           python setup.py test --coverage --testr-args='{posargs}'
+           coverage report --omit=*test*
+           coverage html --omit=*test*
 
 [testenv:docs]
 commands = python setup.py build_sphinx