Remove `[INFO]` prefix from stdout logs

[misohu]

Closes: https://github.com/canonical/data-science-stack/issues/158

• Removing [INFO] prefix from stdout logs. We want to keep it in the logs file.
• Changing docs to reflwct the change
• Adding unit tests for logger

How to test:

git clone <repo>
cd <repo>
pip install .
dss status

The logs in console should not have [INFO]