Document environment variables

[pearu]

Here is a list of environment variables that we should document:

• HEAVYDB_CLIENT_CONF - see rbc/heavydb.py:get_client_config
• RBC_TESTS_FULL - see rbc/tests/heavyai/test_heavyai.py
• OMNISCIDB_DEV_LABEL - see rbc/tests/init.py (requires rebranding)

Originally posted by @pearu in #465 (review)