netcmd: Fix kerberos option for domain backups

netcmd: Fix kerberos option for domain backups

The previous fix still didn't work if you specified --kerberos=yes (in
which case the creds still doesn't have a password).

credopts.get_credentials(lp) should be enough to ensure a user/password
is set (it's all that the other commands seem to do).

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13566

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