Instance creation fails if checkpoint/timeframe are passed

At the staging API, if the client does a request to `POST /project/:id/instance` including `checkpoint_id` and `timeframe_id` in the payload, an instance is created and starts running, but it never gets to the `active=true` state and it is not returned in `GET /project/:id/instance` list. It is possible to get its status at `GET /project/:id/instance/:id`.

cc @ingalls @geohacker