Revert "ktn-base.inc: Use google.com for connectivity check"

This reverts commit ff01a786.

Google is very restrictive and bans IP addresses of our cloud runners in some cases (it thinks they are from Iran or whatever).

The example.com page is owned by IANA and expected to be always available without restrictions. Previous issues with SSL in Ubuntu 16 can not be reproduced today with the latest 16.04 Ubuntu image from Docker hub.

See: https://git.kontron-electronics.de/sw/ked/plan/plan-yocto/-/issues/1250

CC: @estoll @jbauknecht