Default images should be updated over time.

[eduble]

Default images (waltplatform/<node-model>-default) are downloaded from a docker registry (docker hub, most of the time), stored on the server, and then assigned to new nodes and nodes becoming free (walt node release <node>).

After the first node of a given model is detected and cause downloading of this default image, the current software does not allow this image to be updated. It is not a serious problem because backward compatibility is still maintained with future server software.

However, new WALT users also get a clone of these default images at first. Getting newer images, providing more features and an up-to-date OS, would be better for them.

Plan:

1. Try (i.e., do not fail if one of the registries configured is down) to update default images at each start of walt-server-daemon.
2. Provide a subcommand walt advanced update-default-images for explicit update requests.