Flakes test reporter supports reporting github issue in different org

[Flakes test report](https://github.com/knative/test-infra/tree/master/tools/flaky-test-reporter) currently only supports reporting github issues in the same repo where the test takes place, as configured in this [yaml](https://github.com/knative/test-infra/blob/master/tools/flaky-test-reporter/config/config.yaml). As requested on https://knative.slack.com/archives/CCSNR4FCH/p1599021716069900, it's desired to report jobs from orgA as issues on orgB, for example, knative/serving tests create issues in knative-sandbox/net-{} repos

Start in tools/flaky-test-reporter and read the configuration in tools/flaky-test-reporter/config/config.yaml to understand how the current repository is selected. Implement support for reporting tests from one GitHub organization to a repository in another, such as knative/serving to knative-sandbox/net-{}, and verify the reporter still handles same-repository reporting.

Written by the indexing model from the issue text.