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/add_model.py b/modules/libjuju/examples/add_model.py
index 3e46490..0e96fa1 100644
--- a/modules/libjuju/examples/add_model.py
+++ b/modules/libjuju/examples/add_model.py
@@ -19,7 +19,7 @@
 async def main():
     controller = Controller()
     print("Connecting to controller")
-    await controller.connect_current()
+    await controller.connect()
 
     try:
         model_name = "addmodeltest-{}".format(uuid.uuid4())