Jim Ferenczi 9ca33e2450 Add a section named "relations" in the ParentJoinFieldMapper (#25248)
* Add a section named "relation" in the ParentJoinFieldMapper

This commit puts the parent/child definition in an inner section named "relation".
Mapping for the parent-join will look like this:

```
"join_field": {
  "type": "join"
  "relations":
    "parent": "child"
  }
}
```
2017-06-15 14:56:20 +02:00
..