Actions formatting tightened up

Charm now expects a much simpler format for actions.yaml:

```text
<action name>:
  [description]: <string>
  [params]:
    <param 1>: <JSON-schema>
    <param 2>: <JSON-schema>
    ...
  [required]: <string list>
  [other keys e.g. "definitions"]: ...
<next action name>: ...
```
Won't these read better if you define type M map[string]interface{} somewhere before and use it? Not insisting though.