Excluding unit_tests from coverage

Unit tests generally will all be executed, and measuring them will
skew the overall coverage numbers.  Thus, I'm excluding the unit_tests
folder.

Change-Id: I03bf73856dfd850c6c84a82745d0d161b0e9f538

tox.ini

@@ -50,6 +50,7 @@ source =
 omit =
     .tox/*
     */charmhelpers/*
+    unit_tests/*
 
 [testenv:venv]
 commands = {posargs}