dnsupdate: Pass smb.conf through to samba-tool commands

dnsupdate: Pass smb.conf through to samba-tool commands

If you call samba_dnsupdate with a --configfile option, this wasn't
passed through to the samba-tool commands the script tries to run.
Normally, samba_dnsupdate would only be run on the DC itself, so it
shouldn't be a big deal, however, this may be a problem if you install
the samba database into a non-default location (i.e. not
/usr/local/samba).

This patch passes through the smb.conf file, if one was specified.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>