install-script,opensuse: Add omit-xml-declaration to the output format

Without this change the output got from the command-line template will
look like "<?xml version=1> autoyast=device://sda/autoinst.xml" while
what we actually expect is just "autoyast=device://sda/autoinst.xml

".

It's been the faulty part that caused problems with Tumbleweed and Leap
42.2 installations.

Signed-off-by: Fabiano Fidêncio <fabiano@fidencio.org>
Acked-by: Daniel P. Berrange <berrange@redhat.com>