Improve story around cloud-config in ISO

[frelon]

Adding cloud-config to the SeedImage always marshals the cloud-config with the #cloud-config comment, which disqualifies us from putting yip-style cloud-configs in SeedImage and MachineRegistration resources.

Doing customizations in after-install or post-install stage means we have to write an intermediate file to /usr/local/cloud-config/ that will actually be picked up during install process.

Opening this as a discussion to gather feedback and ideas.