Fix getkeytab code to always use implicit tagging.
    
    A mixture of implicit and explicit tagging was being used and this caused
    a bug in retrieving the enctype number due to the way ber_scanf() loosely
    treat sequences and explicit tagging.
    
    The ASN.1 notation used to describe the getkeytab operation uses implicit
    tagging, so by changing the code we simply follow to the specified encoding.
    
    Resolves: https://fedorahosted.org/freeipa/ticket/4404
    
    Signed-off-by: Simo Sorce <simo@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>