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>

install/share/smb.conf.template

@@ -3,7 +3,7 @@ workgroup = $NETBIOS_NAME
 netbios name = $HOST_NETBIOS_NAME
 realm = $REALM
 kerberos method = dedicated keytab
-dedicated keytab file = FILE:/etc/samba/samba.keytab
+dedicated keytab file = /etc/samba/samba.keytab
 create krb5 conf = no
 security = user
 domain master = yes