vyos-hostsd: T4270: resolve only hostname without domain name to 127.0.1.1

This is a fix for commit 665ae50729 ("vyos-hostsd: T4270: do not resolve local
router FQDN to 127.0.1.1") as it made calls to sudo super slow due to:

sudo: unable to resolve host vyos: System error

To avoid the initial issue we only add the hostname without domain name, thus
the FQDN is not resolved by powerdns.

(cherry picked from commit 3712f28025a5bc99e941b5212091a2732b9f6d6c)