rename LLAMASTACK_PORT to LLAMA_STACK_PORT for consistency with other env vars (#744)

# What does this PR do?

Rename environment var for consistency

## Test Plan

No regressions

## Sources

## Before submitting

- [X] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [X] Ran pre-commit to handle lint / formatting issues.
- [X] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
Pull Request section?
- [X] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

---------

Signed-off-by: Yuan Tang <terrytangyuan@gmail.com>
Co-authored-by: Yuan Tang <terrytangyuan@gmail.com>

llama_stack/distribution/server/server.py

@@ -290,7 +290,7 @@ def main():
     parser.add_argument(
         "--port",
         type=int,
-        default=int(os.getenv("LLAMASTACK_PORT", 5000)),
+        default=int(os.getenv("LLAMA_STACK_PORT", 5000)),
         help="Port to listen on",
     )
     parser.add_argument(