fix(cloud-init): truncate all hostnames on first .

[RiRa12621]

truncate all hostnames on first .

This ensures consistent behaviour and only if the hostname still exceeds the character limit, it gets truncated further

How to use

for any hostname, the domain will be cut

Testing done

the extra test case covers a host name with a domain not exceeding the character limit. Regardless of that it is still truncated to the string before the first .

• Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.