Add cloud-init compatibility for Ubuntu VMs

[leona-ya]

• Add cloud-init disk with config files
• update root ssh keys via qemu agent in ensure for ubuntu VMs

PL-133325 and PL-133372

Release process

• Created changelog entry using ./changelog.sh

PR release workflow (internal)

• PR has internal ticket
• internal issue ID (PL-…) part of branch name
• internal issue ID mentioned in PR description text
• ticket is on Platform agile board
• ticket state set to Pull request ready
• if ticket is more urgent than within the next few days, directly contact a member of the Platform team

Design notes

• Provide a feature toggle if the change might need to be adjusted/reverted quickly depending on context. Consider whether the default should be on or off. Example: rate limiting.
• All customer-facing features and (NixOS) options need to be discoverable from documentation. Add or update relevant documentation such that hosted and guided customers can understand it as well. (will be done later, is noted)

Security implications

• Security requirements defined? (WHERE)

• Security requirements tested? (EVIDENCE)

• automated testing (tbd), manual testing, and code review

[ctheune]

Looks great, i'll take over for test coverage and fixing the nixos integration tests.