Consolidate macOS obuilder documentation

Consolidate the description of how obuilder and ocluster worker work for macOS into a single place.

Suggest having obuilder specific pieces exist in [ocurrent/obuilder](https://github.com/ocurrent/obuilder/blob/master/macOS.md) and the deployment of ocluster worker and the details about setting up a macOS worker go into [macos-infra](https://github.com/ocurrent/macos-infra#readme )
