It fixes the mysterious error when all cluster env build is failing when pip uninstall / pip install is written in 2 lines. The root cause will be fixed later
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.
The local environment setup of release tests (in client tests) can sometimes update dependencies of the `anyscale` package to an unsupported version. By re-installing the `anyscale` package after local env setup, we make sure that we can connect to the cluster. Note that this may lead to incompatibilities of the test script, however.
* 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
