Importing code from landscape-client code base is risky

[Free Ekanayaka]

In hooks/common.py we have code that imports directly from landscape-client:

from landscape.configuration import (
    setup, register, LandscapeSetupConfiguration, ConfigurationError,
    stop_client_and_disable_init_script)

The problem with this is that it couples charm and client too much and makes the charm more brittle and sensitive to client changes. Most noticeably this import might break at any time with Python 3 (which is now the Python version that the hooks run with).

This logic should be factored into a script that the client exposes and that the charm can invoke using the subprocess module.