[Samba] Samba Wiki: fix to "Configure DHCP to update DNS records" page

[Ing. Claudio Nicora]

@Rowland Penny: thanks for the BASH script to let DHCP update DNS records (https://wiki.samba.org/index.php/Configure_DHCP_to_update_DNS_records).

I'm suggesting this change:
the "if" condition at line 171 does not use the filename assigned to $keytab variable (at line 41), but a fixed filename (equal to default $keytab filename).
If someone (like me) need to set a different filename, then the Kerberos keytab test fails.

So my suggestion is to replace the "if" condition (around line 171) from

------------------------------
# Check for Kerberos keytab
if [ ! -f /etc/dhcpduser.keytab ]
then
   logger "Required keytab $keytab not found, it needs to be created."
   ...
------------------------------

to

------------------------------
# Check for Kerberos keytab
if [ ! -f "$keytab" ]
then
   logger "Required keytab $keytab not found, it needs to be created."
   ...
------------------------------

Thanks for your work