Allow env variable to set SQLAlchemy echo in SQLAlchemyDatabaseManager
Allow env variable to set SQLAlchemy echo in SQLAlchemyDatabaseManager

SQLAlchemyDatabaseManager.instance() checks a DATABASE_ECHO environment variable and sets the SQLAlchemy engine.echo for the session appropriately. Echo can be set to True, False (default), or debug.

Relates to SL-1109

abn/python-cafe