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.

[s4ke]

Thinking about it, we should rewrite the "list of strings" style into a "list of mappings" style on the fly. It should be rather simple to do and will allow for easier usage by people coming from vanilla stack files which we aim to be compatible with anyways.