Move hook to hooks.py to make it testable.
Return the exit code instead of exiting directly, to make it easier to test the code.
Pass a broker object to the hooks, to make it easier to test them.
Extract landscape and juju functionality into objects, to make them replacable.
Add a simple test for container-relation-joined.
correct function definition.
Add docstrings.
Document the tests.
Add missing parameter.
Remove LandscapeBroker.get_client_config().
No need to pass data_path to the constructor.
s/Testing/Fake/
Drop get_environment_config.
Use read_file.
Use new-style classes.
Typo.
Add a basic test for the container-relation-joined relation.

The purpose of this branch isn't to add full test coverage for the
container-relation-joined relation. It's to make it possible to add
tests for this hooks (and other hooks as well, of course). I added a
simple test to show that it's indeed possible to test it.