Fix region mismatch in XGBoost example

[raykrueger]

What does this PR do?

The Ray on EKS terraform module deploys to us-west-2 by default. The XGBoost example project declared us-east-1 as the region.

This commit moves the variable to variables.tf and defaults it to us-west-2.

Fixes #203

Motivation

It's broke yo

More

• [X ] Yes, I have tested the PR using my local account setup (Provide any test evidence report under Additional Notes)
• Mandatory for new blueprints. Yes, I have added a example to support my blueprint PR
• Mandatory for new blueprints. Yes, I have updated the website/docs or website/blog section for this feature
• Yes, I ran pre-commit run -a with this PR. Link for installing pre-commit locally

For Moderators

• E2E Test successfully complete before merge?

Additional Notes

Tested locally with terraform plan