When pserv requests the pxeconfig from the region, the context nodegroup is not detected correctly.

The fix for this should be twofold:

Fix the pserv code so that the UUID will be sent (sorry, not a proper diff, just notes from my cowboy on the QALab code):
src/provisioningserver/tftp.py:
            # Send the local and remote endpoint addresses.
            local_host, local_port = get("local", (None, None))
            params["local"] = local_host
            remote_host, remote_port = get("remote", (None, None))
            params["remote"] = remote_host
            uuid = 'cbc2e193-262b-4d44-a5ee-b900a4448082' # get the UUID from the config
            params["uuid"] = uuid
            d = self.get_config_reader(params)

The API should use that value when it's provided:
src/maasserver/api.py:
        else:
            # Map from pxelinux namespace subarchitecture names to MAAS
            # namespace subarchitecture names. Right now this happens to be a
            # 1-1 mapping.
            subarch = pxelinux_subarch

        uuid = request.GET.get('uuid')
        if uuid is not None:
            nodegroup = NodeGroup.objects.get(uuid=uuid)
        else:
            nodegroup = find_nodegroup(request)