Feature 10239: Distributed VCA

- Add vca_id in all calls that invoke libjuju. This is for being able to
talk to the default VCA or the VCA associated to the VIM
- Add store.py: Abstraction to talk to the database.
  - DBMongoStore: Use the db from common to talk to the database
  - MotorStore: Use motor, an asynchronous mongodb client to talk to the
database
- Add vca/connection.py: Represents the data needed to connect the VCA
- Add EnvironConfig in config.py: Class to get the environment config,
and avoid LCM from passing that

Change-Id: I28625e0c56ce408114022c83d4b7cacbb649434c
Signed-off-by: David Garcia <david.garcia@canonical.com>