classic image can't get serial with a gadget without a brand store

[Ian Johnson]

Currently, if you create a classic model with a gadget snap and seed it onto an image such as a cloud image, the device is unable to acquire a serial from the store:

```
ubuntu@ubuntu:~$ snap changes
ID Status Spawn Ready Summary
1 Done today at 18:36 UTC today at 18:38 UTC Initialize system state
2 Error today at 18:37 UTC today at 18:37 UTC Initialize device

ubuntu@ubuntu:~$ snap tasks 2
Status Spawn Ready Summary
Done today at 18:37 UTC today at 18:37 UTC Generate device key
Error today at 18:37 UTC today at 18:37 UTC Request device serial

......................................................................
Request device serial

2019-06-28T18:37:11Z ERROR cannot deliver device serial request: Cannot process serial request for device with brand "MY_UBUNTU_ID" and model "test-model"

ubuntu@ubuntu:~$ snap known serial
ubuntu@ubuntu:~$
```

Going through the debug logs for snapd the store is returning 403 (newlines added for clarity):

```
Jun 28 18:44:17 ubuntu snapd[2287]: logger.go:74: DEBUG: < "
HTTP/1.1 403 FORBIDDEN
Content-Length: 324
Content-Type: application/json
Date: Fri, 28 Jun 2019 18:44:17 GMT
Server: gunicorn/19.7.1
Snap-Store-Version: 14
X-Request-Id: SOME-BIG-ID
X-Vcs-Revision: 21acec0
X-View-Name: snapdevicegw.webapi_auth.device_registration

{\"error_list\":[{\"code\":\"device-registration-forbidden\",\"message\":\"Cannot process serial request for device with brand \\\"MY_UBUNTU_ID\\\" and model \\\"test-model\\\"\"}],\"errors\":[\"Cannot process serial request for device with brand \\\"MY_UBUNTU_ID\\\" and model \\\"test-model\\\"\"],\"result\":\"error\"}\n"
```

I reproduced this with the following model.json:

```
{
    "type": "model",
    "authority-id": "MY_UBUNTU_ID",
    "brand-id": "MY_UBUNTU_ID",
    "series": "16",
    "model": "test-model",
    "architecture": "amd64",
    "classic": "true",
    "gadget": "pc",
    "required-snaps": [
        "node-red"
    ],
    "timestamp": "2019-06-25T04:40:53+00:00"
}
```

I can provide more information on how to boot this image to perform a first-boot with the seed and reproduce it (I did this with qemu). It's a bit more involved and this bug is just to explain the root issue with not getting a serial.

[Ian Johnson (anonymouse67)]

This issue also prevents any gadget installed in the image from processing the auto-connection rules specified in the `connections` stanza of the gadget.yaml.

[Zygmunt Krynicki (zyga)]

I believe this is going to be addressed by improvements to the snap store and the serial vault, where a brand may get a serial assertion for a given model from the generic / shared serial vault. Having said that I'm not aware of the status of this work. I'm marking it as confirmed though.