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>: ...
```
Looking further down it seems most of the actionSpecs used are the same apart from the properties field. How about adding a helper:
testActionSpec(properties map[string]interface{}) *ActionSpec {
    return &ActionSpec{
Description: "Take a snapshot of the database.",
Params: map[string]interface{}{
- "title": "Snapshot params",
+ "title": "snapshot",
"description": "Take a snapshot of the database.",
"type": "object",
"properties": properties,
"required": []interface{}{"outfile"}}}
}

Or something along these lines - ISTM this will drastically reduce the number of repeated lines.