pkg_in_pipe: generate a draft of the release post from the report

[glehmann]

Add release_post.py, which writes the "What changed" and "Versions"
sections of the XCP-ng release post from the json report, with the
change descriptions for the users taken from the related pull requests.

Signed-off-by: Gaëtan Lehmann gaetan.lehmann@vates.tech

This PR is part of a tree containing 4 PRs:

1. master
2. pkg_in_pipe: keep PRs merged by rebase or squash #838 → master
3. pkg_in_pipe: write a machine readable version of the report #839 → pkg_in_pipe: keep PRs merged by rebase or squash #838
4. pkg_in_pipe: move the plane cards of the builds of a tag to a given state #840 → pkg_in_pipe: write a machine readable version of the report #839
5. "pkg_in_pipe: generate a draft of the release post from the report" (this PR) → pkg_in_pipe: write a machine readable version of the report #839

[gduperrey]

Is it mandatory to run the scripts in a Docker container?
Can't we just run them directly on a machine?

[glehmann]

Is it mandatory to run the scripts in a Docker container?
Can't we just run them directly on a machine?

No docker required, it runs fine locally