Feature 8681: Add VCA HA capabilities

- juju watcher: fix exception handling when websocket is closed
- libjuju.py:
  - Get vca endpoints from database (if exists)
  - Update vca endpoints in database
  - Get controller per operation (remove controller from class attribute)
  - Add health check: Every 5 minutes the connection to the controller will be tested.
- Dockerfile: update it to 18.04

Change-Id: Ifa0a3a705d48fd736a9634b0722bdc914003b497
Signed-off-by: David Garcia <david.garcia@canonical.com>