Add unit tests to increase coverage

This patch proposes adding several new unit tests in effort to increase
the total unit test coverage.

Additionally, the ./tempest/serial_tests/ directory is not included in
coverage calculation. The reason being we don't want to test tests with
unit tests, which becomes a bit of an overkill.

Finally, there is an addition of a new option for the coverage program
called --fail-under. It will result the program in failure if the
coverage is under set percentage. Reason for this implementation is to
encourage writing unit tests for new code.

Change-Id: I804116413cd7d73cd7e5ae71409a8855ef937b88
diff --git a/tox.ini b/tox.ini
index 47ef5eb..fa32ba3 100644
--- a/tox.ini
+++ b/tox.ini
@@ -64,7 +64,7 @@
   coverage combine
   coverage html -d cover
   coverage xml -o cover/coverage.xml
-  coverage report
+  coverage report -m --fail-under=76
 
 [testenv:debug]
 commands = oslo_debug_helper -t tempest/tests {posargs}