Remove the cloud-init package during server installation

[rmol]

Status

Ready for review

Description of Changes

Towards #5663

We don't want Ubuntu's cloud-init package on the servers, not least because it can make them take ten minutes or more to boot, apparently while waiting for systemd-random-seed to initialize.

Testing

1. Prepare two fresh Ubuntu Focal servers.
2. On your admin workstation, check out this branch and run the installation:
   • git fetch --all
   • git checkout -b remove-cloud-init origin/remove-cloud-init
   • ./securedrop-admin install
3. Observe that the installation does not fail because of timeouts waiting ten minutes for the servers to reboot.
4. Verify that cloud-init is no longer installed on either server.
5. Run the configuration tests:
   • ./securedrop-admin tailsconfig
   • ./securedrop-admin verify
   • The common/test_system_hardening.py::test_unused_packages_are_removed test should pass.

Deployment

We don't have cloud-init on existing Xenial instances, and this change means we also won't on new Focal systems.

Checklist

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR

Choose one of the following:

• I have opened a PR in the docs repo for these changes, or will do so later
• I would appreciate help with the documentation
• These changes do not require documentation

[rmol]

More findings:

• I think it's the Seed Random module of cloud-init that's causing the hang.
• It is also cured by our 5.4.88-grsec kernel, presumably because of the new boot-time entropy collection in 5.4.

Just trying to document for posterity, since we don't want cloud-init at any rate, but should we ever need it, it should be fine as long as we're on a Focal-era kernel.

[emkll]

Thanks @rmol this addressed the issue for my NUC, but not on Mac Mini. As you identify in #5771 (comment), running a 5.4 series kernel fully resolves the issue on all hardware I've tested against. Regardless, cloud-init is something we want to remove from Focal SecureDrop instances.

I do have other testinfra tests failing on a Focal install, but unrelated to this PR.