Roles for the build server and the edge appliance

The add-a-machine guide suggested appliance-builder, camera-stream-source and edge-nvr as examples. They were registered unchanged on 6 October because they name responsibilities, not machines: the build server holds the first two today and could lose either; the appliance holds the third because the product installed it as one.

Not added: a role for the webhook receiver (it is a service of the build server, not a responsibility of the machine) and development-worker for the build server (it builds, but it does not run the isolated task lifecycle that role describes on Shipyard).