Untangle the use of environment variables throughout

[rustprooflabs]

Details

The heavy use of environment variables is a legacy artifact of the pre-Docker image days. I don't think they should generally be required anymore, but they're everywhere and used in ways even I cannot fully remember.

Example

From db.py:

    with get_db_conn(conn_string=os.environ['PGOSM_CONN']) as conn:

Why doesn't that use connection_string() in the same module? Dunno yet.