Repository layout: adopt a standard main branch will 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 master 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 master branch

My proposal is to move to a standard structure with a master branch and have a first level directory representing the OS version the templates must be used for, e.g. el7x-x86_64, el8x-x86_64, el8x-aarch64...

Addressing this issue requires:

• Renaming the most recent branch to main
• Import the other active branches, preserving their history. https://github.com/hraban/tomono may be a useful tool for achieving this.
• Adapt release tools to the new (simpler) layout
• Update the repository structure description in get-library-template