OSS release tests currently run with hardcoded Python 3.7 base. In the future we will want to run tests on different python versions.
This PR adds support for a new `python` field in the test configuration. The python field will determine both the base image used in the Buildkite runner docker container (for Ray client compatibility) and the base image for the Anyscale cluster environments.
Note that in Buildkite, we will still only wait for the python 3.7 base image before kicking off tests. That is acceptable, as we can assume that most wheels finish in a similar time, so even if we wait for the 3.7 image and kick off a 3.8 test, that runner will wait maybe for 5-10 more minutes.
Passing tests: https://buildkite.com/ray-project/periodic-ci/builds/2560#_
Add an echo timestamp to the post build commands of the ray lightning release tests to trigger a cluster env rebuild and get the latest versions of ray lightning. Without this, the cluster env gets cached so an outdated version is installed on the cluster that is different than the one on the driver, resulting in the below failures.
Closes#21871Closes#21863
Also reinstalls the dependencies in the post build commands so old versions are not cached in the Docker images
* use nightly
* switch ml cpu to ray cpu
* fix
* add pytest
* add more pytest
* add constraint
* add tensorflow
* fix merge conflict
* add tblib
* fix
* add back uninstall
