Provision virtual machines

I need three identical Ubuntu virtual machines for the new staging cluster, each with 4 vCPU, 16 GB RAM, and 200 GB SSD, networked on the same VLAN and reachable from the CI server. Create them in the next three hours, apply the latest security kernel patch, add me and the dev lead to sudoers, and hand me their IPs and SSH fingerprints so I can integrate them into our automation.

Before I hand these VMs to the devs, make them obvious and safe to use: ensure cloud-init sets a default user and SSH key, put the patch level and kernel version at the top of /etc/os-release, include a one-paragraph README in /root explaining how to reach the CI artifacts, and highlight any nonstandard package you installed that might surprise an app reviewer.

I created three Ubuntu VMs with the requested resources and applied the kernel patch, but the CI server cannot SSH in; network tests from the host to the VMs work and the VMs resolve DNS. I am afraid I missed a firewall rule or cloud tenant security group and cannot tell if the issue is on the host, the tenant network, or a VM-level service. What is the most likely cause and the fastest next checks to restore CI access?

Every time we provision staging machines, we waste an hour fixing users, packages, and SSH keys because the golden image drifts or documentation is out of date. I want one habit change that stops this recurring rework across teams. Which single step saves the most time and reduces mistakes: enforce immutable golden images, add a post-provision validation script, or require a checklist sign-off, and how do we implement it so the devs accept it?