Add option to pass in openj9 systemtest repo and branch

Currently we are using the default [eclipse-openj9/openj9-systemtest ](https://github.com/eclipse-openj9/openj9-systemtest) to run against openj9 java implementation. It would be nice to add eclipse-openj9/openj9-systemtest repo and branch as options. So run-aqa action can also be used as the part of eclipse-openj9/openj9-systemtest developer's PR tests.

Start by locating the run-aqa GitHub Action entry point that currently uses the default openj9-systemtest repository and branch, then trace how action options are defined and passed through. Done means callers can override both values for developer PR tests while the existing defaults continue to work.

Written by the indexing model from the issue text.