install: do not assume /etc/krb5.conf.d exists
    
    Add `includedir /etc/krb5.conf.d` to /etc/krb5.conf only if
    /etc/krb5.conf.d exists.
    
    Do not rely on /etc/krb5.conf.d to enable the certauth plugin.
    
    This fixes install on platforms which do not have /etc/krb5.conf.d.
    
    https://pagure.io/freeipa/issue/6589
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Lukas Slebodnik <lslebodn@redhat.com>