[deepspeed] Enable multiple test runs on single box, defer to DS_TEST_PORT if set
# What does this PR do?
DeepSpeed currently runs the HF/DS integration tests in our CI for every PR we get. We are attempting to co-locate some of our test runners on single long running nodes. This PR will help us run multiple tests on the same node in parallel by allowing the torch.distributed port to be defined in an environment variable. This PR should work as it did previously if the variable is not set.

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@stas00 
