Khepri: Clean up the setup/clustering code of the integration code

[Why]
The `rabbit_khepri` module grew during the work to add Khepri support to
RabbitMQ and while Khepri was itself written. The current code is
therefore unorganized.

[How]
This commit tries to sort the code that manages the setup of Khepri and
the functions tha deal with the Khepri cluster. It also groups functions
which provide support for CLI commands.

It also adds documentation to several functions.

Finally, when a node joins a cluster, we stop displaying the content of
the Khepri tree.

Author: dumbbell