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.
horovod_user_test_master is failing with recent horovod release[[link](https://buildkite.com/ray-project/periodic-ci/builds/2960#61dabda8-eea0-4b7b-93bf-9e341926d3fd)].
Error message is saying:
```
AttributeError: Can't get attribute '_ExecutorDriver' on <module 'horovod.ray.runner' from '/home/ray/anaconda3/lib/python3.7/site-packages/horovod/ray/runner.py'>
```
The horovod test is set up in such a way that it has the "driver" (a.k.a. client) part (which is the code that runs in a buildkite agent) and the "cluster" (a.k.a. server) part (which runs in Anyscale cluster). Driver's dependency is specified by `release/ml_user_tests/horovod/driver_setup_master.sh` while cluster's dependency is specified by `release/horovod_tests/app_config_master.yaml`.
The two communicate via Anyscale client.
The above error message is complaining that while client's horovod version has _ExecutorDriver in runner.py, the server's horovod doesn't. This is due to the version mismatch of the above two files. This PR brings the two horovod dependency to both point to horovod master.
* 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
