use 'ostree admin set-origin' to manipulate origin files/info

@dustymabe had a lab at Flock where I learned of this magic:

```
# ostree admin set-origin --index 0 local http://localhost:8000/ostreerepo/ fedora/26/x86_64/updates/atomic-host
# systemctl restart rpm-ostreed.service
```

We can use this more natural way of modifying the origin file rather than the hacky sed/regexp way we do now.