system hooks for group creation / deletion don't contain owners
### Summary
The [system hooks](https://docs.gitlab.com/ee/system_hooks/system_hooks.html) `group_create` and `group_destroy` don't contain owner information. The examples in the documentation still do.
### Steps to reproduce
- set up [system hook](localhost:3000/admin/hooks)
- create or delete group
### What is the current *bug* behavior?
Data sent by the hook:
```json
"owner_name":null,
"owner_email":null
```
### What is the expected *correct* behavior?
An array of owners (because there can be multiple) is sent:
```json
"owners": [{
"id": 41,
"name": "John Smith",
"username": "johnsmith",
"email": "johnsmith@gmail.com"
}, {
"id": 42,
"name": "Maria Smith",
"username": "maria"
}]
```
Users without a public e-mail should probably not have the email included.
### Possible fixes
Use `model.owners` instead of `model.owner` in [`SystemHooksService`](https://gitlab.com/gitlab-org/gitlab-ce/blob/v10.1.1/app/services/system_hooks_service.rb#L46-55)
issue