[services] Backup - Set backup id in create new backup reactor's response

Summary:
Depends on D3596

Setting the backup id for the new backup in the reactor's response is missing, this diff fixes it.

Test Plan: The same as in D3535

Reviewers: palys-swm, geekbrother, varun, jimpo

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3610