Group, List and Affix settings generate a bogus diff that turns the actual
diff into a string containing a json structure for instance:
```
"action" : {
"search" : {
"remote" : {
"" : "{\"my_remote_cluster\":\"[::1]:60378\"}"
}
}
}
```
which make reading the setting impossible. This happens for instance
if a group or affix setting is rendered via `_cluster/settings?include_defaults=true`
This change fixes the issue as well as several minor issues with affix settings that
where not accepted as valid setting today.
9809760eb0