bash/staging: prune unexpected services

[spiffxp]

Related:

• Part of: audit followup: k8s-staging-* projects should not have misc services enabled #1675

Specifically

• add a mechanism for staging projects to specify special case services
  that they expect to have enabled
• use that mechanism for cluster-api-gcp and sig-storage
• set obnoxious env var that causes ensure_only_services to disable any
  service not explicitly requested as enabled
• cleanup: order special case functions by project name

Last step before I call #1675
done will be to run ensure-staging-storage.sh for all staging projects
just to ensure we've pruned all unexpected services.

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[spiffxp]

I've run ensure-staging-storage.sh cluster-api-gcp and ensure-staging-storage.sh kubetest2 to ensure this works for both projects that have a stock set of services, and projects that have additional services enabled as a special case.

[ameukam]

/lgtm