Actions formatting tightened up

Charm now expects a much simpler format for actions.yaml:

```text
<action name>:
  [description]: <string>
  [params]: <map>
  [required]: <string list>
  [other keys e.g. "definitions"]: ...
<next action name>: ...
```
Merge pull request #81 from binary132/actions-fix

 Actions formatting tightened up