RHDH Prow test entries and cluster pools

Read and change which platform versions RHDH is tested on in openshift/release. Listing and coverage analysis work against the remote repository through gh; generation and removal need a local checkout because they end in make update.

Run the bundled scripts instead of reconstructing their YAML logic. Every script accepts --repo-dir to point at a local openshift/release checkout and falls back to the GitHub API when it is absent.

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