Clean up Presets

[irbekrm]

At the moment we have a bunch of presets in ./config/jobs/cert-manager/config.yaml and a bunch more in ./config.yaml

We should:

• place all the presets as close as possible to jobs that need them, i.e ones that are for a specific repository in the directory where jobs for that repo live
• document that approach in https://github.com/jetstack/testing#creating-new-prowjobs
• delete the presets that are no longer used