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
