text: Title: Bug: For external Executors, passing entire address to `host` does not work
  Body: The following syntax, which is supported for the gateway, does not work for external executors:
  
  ```python
  f.add(host='grpc://localhost:1234', external=True)
  ```
  
  Instead, host and port have to be precised separately:
  
  ```python
  f.add(host='localhost', port=1234, external=True)
  ```
  
  Not that this bug also applies to replicated external Executors:
  
  ```python
  f.add(host='grpc://localhost:1234,grpc://localhost:1235', external=True)
  ```
  