This PR adds concurrency groups to Buildkite release test runs with new release test package. Five concurrency groups are defined (large-gpu, small-gpu, large, medium, small). If not specified manually, concurrency groups are inferred from used cluster resources.
Example pipeline: https://buildkite.com/ray-project/release-tests-branch/builds/55#09109eac-d22e-43bc-889e-078cfb037373 (click on Artifacts --> pipeline.json)
