Repository layout: adopt a standard main branch with all the supported versions

[jouvin]

For reasons I forgot, when we created this repo at SVN -> Git migration time, we opted for a branch per OpenStack version and no main branch. It turned out not to be a great idea: it makes everything more complicated, including:

• A release must tag several branches
• Fixing a problem affecting several middleware versions requires several PRs
• get-template-library must do complicated things to select all the branches to checkout
• Obsoleting a branch requires renaming it with a .obsolete suffix rather than removing the corresponding set of templates in the main branch

My proposal is to move to a standard structure with a master branch and have a first level directory representing the OpenStack version the templates must be used for, e.g. antelope, caracal ...

As this repository is not part of the Quattor release and not (yet) managed by get_template_library, this change will have a very low impact and I will start it with the templates for the antelope version. Templates for previous versions, if needed, will have to be retrieved from the corresponding branch rather than main.

[jrha]

Previous version branches can have a commit added to move them to a sub-directory and then also be merged into master.