gitlab-rulez: Ensure multiple branches at the same time

The current way to handle ensured branches didn't work well in this quarter as we had to create two branches at the same time, the apertis/v2019 and apertis/v2020dev0 ones, both based on apertis/v2019pre.

For this reason, ensure_branch has become ensure_branches and now accepts an array of objects, with name and base as before plus a required field to mark the unavailability of the branch or of its base as an error.