allow setting DATABASE_PASSWORD in the environment
    
    Renamed the setting to DATABASE_URI, with the password filled into the
    URI from the DATABASE_PASSWORD environment variable if that is defined.
    
    The SQLALCHEMY_DATABASE_URI setting is now just used internally within
    the app to pass the complete URI down to Flask-SQLAlchemy.
    
    This change is for OpenShift, which does not have any simple way to
    inject secrets into configuration files. It is simpler to make the
    configuration be non-secret, and pass secrets in as environment
    variables.