[feature-request] Expose pytest options in testrunner.py as CLI options

[kace]

Checklist

• [x] I've prepended issue tag with type of change: [feature]
• [ ] (If applicable) I've documented below the DLC image/dockerfile this relates to
• [ ] (If applicable) I've documented the tests I've run on the DLC image
• [ ] I'm using an existing DLC image listed here: https://docs.aws.amazon.com/deep-learning-containers/latest/devguide/deep-learning-containers-images.html
• [ ] I've built my own container based off DLC (and I've attached the code used to build my own image)

Concise Description: It would be useful to expose some pytest options in the testrunner.py script.

In my particular case I wanted to disable the rerun functionality that prevents running tests that have already passed in previous test runs. This could be done by commenting out https://github.com/aws/deep-learning-containers/blob/master/test/testrunner.py#L376, but ideally wouldn't require a code change. Another useful example would be the ability to specify test timeouts.