This PR reduces the concurrency limit. Based on the back of envelope calculation, the current concurrency limit can easily exceed the service quota.
Given large == 2048 vCPUs, it will use about 20K vCPUs, which is slightly larger than the limit.
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)
