Use spot instances for chaos tests.
We can also experiment with other tests that don't suppose to have dead nodes, but let's do it once the nightly infra is stabilized
This PR is mostly for implementing "fixture" for nightly test. Note that the current fixture implementation is not that great, and we can probably improve this in the future after refactoring e2e.py.
* 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