This commit finishes the first step for migrating our complete build and
release pipeline to Concourse CI.

We're creating here a new "Release" jobs group in our pipeline.
We can now release Milestones from our pipeline with:

* the "stage-milestone" job, which creates and tags a new Milestone
version on the source repository, builds the artifacts and deploys them
in a staging repository and updates the version to the next SNAPSHOT.

* the "promote-milestone" fetches the build information from the staging
repository and promotes the released milestone in the official milestone
repository.

Interactions with the Artifactory and Bintray REST APIs are done using
the https://github.com/spring-io/concourse-release-scripts project.

Closes gh-22490