Fix for renaming DBInstance (See #1529).

Sample usage:
    ===========================
    api_version = '2013-05-15'
    conn = boto.rds.RDSConnection(aws_access_key_id=aws_id, aws_secret_access_key=aws_key,api_version=api_version)
    conn.modify_dbinstance(
        id='oldIdentifier',
        new_id='newIdentifier',
        apply_immediately=True
    )