Create tooling to checkout all OpenWhisk repositories

OpenWhisk project consist of multiple git repositories (43 currently). It would be good to have some tooling which allows a developer to checkout all the repositories. I felt a need for such a tooling for apache/incubator-openwhisk#3277

Sometime back we did similar exercise in Apache Sling (200+ repos). There we have a single [aggregator project][2] which can be used to checkout all Sling projects. We can use a script like [this][3] (by @rombert) to generate list of all OpenWhisk repos

Plan here is to use the [Google repo][1] tooling along with a custom script to generate the list of all OpenWhisk repos which can be used to perform the sync

[1]: https://android.googlesource.com/tools/repo
[2]: https://github.com/apache/sling-aggregator
[3]: https://github.com/apache/sling-aggregator