Create branch and pipeline for Puppet Server 2.1 work

We need to get a separate branch and pipeline setup for Puppet Server 2.1 development work since it will be some number of days before we're ready to cut the Puppet Server 2.0 GA release and don't want to hold up 2.1 development. Once the 2.1 branch is available, we should be able to get started on SERVER-530, merging up the stable branch from the 1.0.8 tag into the 2.1 branch.

While the 2.1 branch exists, any new work that comes into the master branch before 2.0 GA should periodically be merged up to the 2.1 branch.

The 2.1 branch will hopefully be fairly short-lived. Once the 2.0 GA release is cut from the master branch, we should be able to fast-forward merge content from the 2.1 branch back to the master branch and then delete the 2.1 branch and CI pipeline.

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This ticket will involve the following:

1) Submit a PR with changes to the README_BRANCHING.md doc, covering the 2.1 development branch strategy. A separate PR to the master and stable branches should be filed.

2) After the above PR is merged to master, create a "2.1" branch in the Puppet Server repo - branched from whatever the HEAD of the master branch is at the time.

3) Work with QE to get a CI pipeline (ci-job-configs, etc.) set up for the 2.1 branch. That pipeline should basically draw from the current state of the pipeline on the master branch.