Release management

We have multiple live versions of CitC. This is required since if a cluster was created against, e.g. CitC 2 the infrastructure and configuration of the mgmt node will require all compute nodes to be created in a compatible state. Therefore we must maintain a branch in this Ansible repo as long as there may be corresponding installations.

To make his work we have a git branch in the Ansible repo which is named for the major version of the software, e.g. "2". As long as a change being made is compatible with the first commit on that branch then we continue with that version.

Once we need to make a breaking change, we merge the version branch (e.g. 2) into master and then make a new branch from master called, e.g. 3 and continue there.

To ensure that PRs etc. are being made against the correct branch we set the GitHub default branch to the latest version branch.