ts-debian-*-install: Replace dots in hostnames by dashs

ts-debian-*-install: Replace dots in hostnames by dashs

When running ./ts-debian-di-install, hostname on the command line is
interpreted by the debian installer. As the installer find it to be a
FQDN, it uses part of the hostname as the domain, thus overwriting the
value from the DHCP and from d-i netcfg/get_domain setting (maybe).

But the result is that /etc/resolv.conf contains "search
bookworm.guest.osstest" and can't find an IP for "cache". So the
installation fails.

Also replace ".guest.osstest" in a few other places, even if it
may not be an issue.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>