adtrust: remove FILE: prefix from 'dedicated keytab file' in smb.conf
    
    Samba 4.5 does not allow to specify access mode for the keytab (FILE: or
    WRFILE:) from external sources. Thus, change the defaults to a path
    (implies FILE: prefix) while Samba Team fixes the code to allow the
    access mode prefix for keytabs.
    
    On upgrade we need to replace 'dedicated keytab file' value with the
    path to the Samba keytab that FreeIPA maintains. Since the configuration
    is stored in the Samba registry, we use net utility to manipulate the
    configuration:
    
        net conf setparm global 'dedicated keytab file' /etc/samba/samba.keytab
    
    Fixes https://fedorahosted.org/freeipa/ticket/6551
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>