Ensure proxy and server images are only provided with --use-pathways.

[RoshaniN]

Fixes / Features

• Remove default values for proxy-server and server images.
• Ensure user provides both proxy-server-image and server-image when --use-pathways is set and vice-versa.
• Validate that the proxy-server-image and server-image exist on GCR.
• Remove a Pathways flag.

Testing / Documentation

Testing details. Additionally, I did some manual tests to see that the user gets the right error message for -

[XPK] --proxy-server-image and --server-image need to be provided with --use-pathways.

Also manually tested that invalid docker images are detected and user is guided with an error message.

[XPK] Failed to validate your docker image, check the docker image. You should be able to navigate to the URL gcr.io/hello_world in cloud-tpu-multipod-dev
[XPK] Please check the proxy-server-image or server-image as advised!

• [ y ] Tests pass
• [ y ] Appropriate changes to documentation are included in the PR

[RoshaniN]

I will be punting this PR for later. With the way parsers are configured, it is not straightfoward to ensure --use-pathways is provided when any of the other Pathways args are provided.

I will resume when the requirements are clearer and this edge case takes priority.