[RFE] Allow re-run of existing cross repo

Generally speaking, when using cross_repo_test, you aren't going to nail it on the first go.

Unfortunately with the current implementation of cross_repo_test, @miq-bot owns the PR, so it is the only one that re-trigger the build.

The feature here would be to allow a way of retriggering the cross repo build so that you can test again with the latest commits.

Option #1

A first pass might just be adding a new command that looks like this:

@miq-bot cross_repo_retest ManageIQ/manageiq-cross_repo-tests#1234

And the bot will close/open that PR, re-triggering the cross repo tests. This doesn't address making changes to what is being tested, so some further thought for how to make additions to that would need to be considered.

Option #2

Re-use cross-repo-tests, but allow for no-args. If a previous cross-repo-tests command is detected, then it will just use that PR.

@miq-bot cross_repo_tests

This would also allow for adding additions or subtractions if needed, as you could just take the original command, and add/subtract from it. In this form, it would just re-write the existing commit if a PR is already detected.