def get_version(opts, app):
    """Get version of a specific app

    Args:
        opts (dict): Nephos options dict.
        app (str): Helm application name.

    Returns:
        str: Desired version of Helm app, if specified. Defaults to None.
    """
    if "versions" in opts and app in opts["versions"]:
        return opts["versions"][app]
    else:
        return None