Make openapi-generator run in container and with collected git repos

[pedro-psb]

Currently, the openapi-spec generation uses plugin packages from pypi.
This causes an inconsistency with the exact checkout of each plugin.
Thus, we want it to use exactly the git checkout we are using for building the docs.

Also, it creates venvs for each plugin and relies on system dependencies to be available.
We should instead use the pulp base-image (which contains system dependencies) and install all plugins at once from the git checkouts.