Charm actions fix

Depends on https://github.com/juju/charm/pull/81.

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>: ...
```

This PR fixes tests on juju/juju to reflect this change.