Make HcfMiddleware._start_job method return job id and make it public

Motivation is to make new job ID available when overriding start_job method. This is useful if, for example, you want to add some tags to newly created job.

Unfortunately I failed to run tests locally and as a result  - write test for this change. It seems like I should have service for testing running in background (hubstorage mock?):

```
ConnectionError: HTTPConnectionPool(host='localhost', port=8003): Max retries exceeded with url: /hcf/2222222/test/s/0 (Caused by <class 'socket.error'>: [Errno 111] Connection refused)
```

If you explain me how can I run tests locally - I could write one and add to this PR.