Introduce a CLI object

Instead of writing the CLI calls to stderr as INFO loglines, now add
them to a list of messages by prune/create/sync.

Always close the VPP connection after finishing, by adding a destructor
to the VPPApi class.

At the end of each phase, print out what was gathered to stdout. Later,
I will make this more advanced (output to file, output directly to a
pipe of a running vppctl binary, etc).

vpp/vppapi.py

@@ -56,6 +56,7 @@ class VPPApi():
         if not self.connected:
             return True
         self.vpp.disconnect()
+        self.logger.debug('Disconnected from VPP')
         self.connected = False
         return True