Fix deprecation import for flask sqlalchemy
    
    Import from flask.ext.sqlalchemy has been deprecated. This fix is
    helpful to supporess the deprecation warning and avoid import error once
    newer flask_sqlalchemy version 2.3.2 is installed, in which version
    flask.ext.sqlalchemy is replaced with flask_sqlalchemy.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>