Make external hostname in executor optional #76
Description
Having the ability to set an external/advertised hostname is great since it provides users a lot of flexibility in network deployments. However, having it a required argument is a pain for the most common scenario, where the scheduler, client and executors talk to each other in the same network (e.g. k8s or docker-compose).
We should make the external hostname optional. If the scheduler receives executor metadata without a hostname, it should register the caller's IP address as the hostname.
This will make it easier to deploy the executors as a kubernetes deployment, or to docker-compose scale ballista-executor= in the integration tests.