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>: ...
```
Or even:

```go
for i, test := range []struct {
  should string
...
}{{
  ...,
}, {
..
}} {
 c.Logf ...
}
```