Server Upgrade: Allow base64 encoded values
    
    This patch allows to use base64 encoded values in update files.
    
    Double colon ('::') must be used as separator between attribute name
    and base64 encoded value.
    
    add:attr::<base64-value>
    replace:attr::<old-base64-value>::<new-base64-value>
    
    https://fedorahosted.org/freeipa/ticket/4984
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>