New N2VC interface + updated libjuju

This commit introduces the Python3 N2VC module, which acts as a standard
interface to the VCA.

The goal of this is to provide a common way for modules to interface
with the VCA.

- Updated libjuju from 0.6.1 to 0.7.3

Signed-off-by: Adam Israel <adam.israel@canonical.com>
Change-Id: Ide70fb5ae5797eb6486de24653dc09a23f9c009e
diff --git a/modules/libjuju/examples/leadership.py b/modules/libjuju/examples/leadership.py
index b231003..dbd1b6e 100644
--- a/modules/libjuju/examples/leadership.py
+++ b/modules/libjuju/examples/leadership.py
@@ -13,7 +13,7 @@
 
 async def report_leadership():
     model = Model()
-    await model.connect_current()
+    await model.connect()
 
     print("Leadership: ")
     for app in model.applications.values():