Node pre-registration

[edufschmidt]

As a user, I would like to be able to (pre-)register nodes so that they can be configured (i.e, join networks, connect to peers, etc) before they go online.

The steps below should serve as a starting point:

1. User requests the pre-registration of a new node, either via CLI or web UI.
2. A node is created in the server, as if it had automatically registered, and a secret S is returned.
3. The node can be arbitrarily configured, as if it were a regular node.
4. The secret S is then passed to a client node, via configuration file, flag, etc.
5. When a client node tries to register to the server and presents the secret S, it is immediately associated with the pre-registered client, and the secret is rendered useless.