docker-sdp support configs/secrets as list of strings

[s4ke]

This does not work as it crashes the current code that expects a dictionary:

secrets:
   my_secret:
      file: /some/path

services:
   my_service:
      secrets:
         - my_secret

Instead one has to write

secrets:
   my_secret:
      file: /some/path

services:
   my_service:
      secrets:
         - source: my_secret
            target: my_secret

which will not work with the secret approach we currently take.

While we could rewrite the list to a dictionary, this feels like a bit much rewriting so we should instead throw a proper error that lets the user know that they should rewrite their yaml slightly.