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/tox.ini b/tox.ini
index bc316e1..a0f0503 100644
--- a/tox.ini
+++ b/tox.ini
@@ -27,9 +27,9 @@
 commands =
   coverage erase
   nose2 -C --coverage n2vc --plugin nose2.plugins.junitxml -s n2vc
-  coverage report --omit='*tests*'
-  coverage html -d ./cover --omit='*tests*'
-  coverage xml -o coverage.xml --omit='*tests*'
+  coverage report --omit='*n2vc/tests*'
+  coverage html -d ./cover --omit='*n2vc/tests*'
+  coverage xml -o coverage.xml --omit='*n2vc/tests*'
 
 [testenv:pylint]
 basepython = python3