public-clouds.yaml needs automation and verification

[Curtis Hovey]

Human's can publish new public-clouds.yaml by running jobs in specific order for a selected file. We need a way full proof way to select a public-clouds.yaml and publish it.

If there are test results for changes to clouds/fallback-public-clouds.yaml, we will have confidence about which version to select. The results need to be for supported and future Jujus.

Publication need some verification that the file arrived and is working. A bootstrap for the changes regions would suffice.