Add unit tests to N2VC refactor

This commit adds unit tests for the following
modules:
juju_watcher.py 96% coverage
libjuju.py 72% coverage
utils.py 100% coverage

Minor libjuju.py fixes found with unit testing
Fix testing with tox

Change-Id: I9f23ce2f18aac6765edfa955ed200802c27d9047
Signed-off-by: Dominik Fleischmann <dominik.fleischmann@canonical.com>
diff --git a/n2vc/exceptions.py b/n2vc/exceptions.py
index 59bcc1a..256860e 100644
--- a/n2vc/exceptions.py
+++ b/n2vc/exceptions.py
@@ -33,6 +33,14 @@
     """The Application cannot be found."""
 
 
+class JujuLeaderUnitNotFound(Exception):
+    """The Application cannot be found."""
+
+
+class JujuActionNotFound(Exception):
+    """The Action cannot be found."""
+
+
 class JujuMachineNotFound(Exception):
     """The machine cannot be found."""