With this change, the location of the secret SSH key can be configured
through `${RAD_HOME}/config.json` so that the node key does not have to
be placed under `${RAD_HOME}/keys` anymore.
Further, there is now an option to override `config.json` directly when
executing `radicle-node` via the command line argument
`--secret`.
The primary motivation is more flexible deployments, for example
leveraging external secret management solutions, like
<https://systemd.io/CREDENTIALS/>.
The secret key is fingerprinted by taking the fingerprint of the public
key corresponding to the secret key. This allows to detect when the
secret key changes.
Lorenz Leutgeb