[batch] Clean up environment variables for easier local execution of batch tests

[daniel-goldstein]

Trying to make it more ergonomic to simply do python3 -m pytest batch/test/test_batch.py::test_job (now works without any extra environment variables or configuration).

This involved the following changes:

• Deleted of some env vars that are no longer used / can be easily consolidated into existing ones
• Gave defaults to those testing env variables for which there are reasonable defaults. E.g. DOCKER_ROOT_IMAGE and HAIL_GENETICS_HAIL_IMAGE.
• Pushed other environment variables for which there are not reasonable defaults into the tests that need them. If you run a test that requires HAIL_CLOUD, you'll still get an error that that env variable is unset and you should set it. But, if you just want to run a single test that doesn't need HAIL_CLOUD it won't get in the way.

[danking]

This is great!