SUDO: be able to parse modifyTimestamp correctly
    
    We were unable to parse modifyTimestamp where a non-numeric part
    (timezone) was involved. The format is YYYYMMDDHHmmssZ. It may
    also contain fraction or different timezone, everytime separated
    from the datetime by character. This patch gets the numberic part
    and then appends the string part again to get value usable in filter.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2970
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>