glcli: add node init

[Lagrang3]

Add "node init" command to initialize the local data directory for the greenlight node. Including the generation of the hsm_secret file either from user provided mnemonic phrase or by generating a new one. The user is shown on screen the mnemonic phrase from which the secrets are derived.

Replaces an earlier attempt to incorporate a secret generation tool in glcli: #657

The idea is that a user uses this command to generate the secret before registering his greenlight node.
For example

$ glcli node init
Local greenlight directory created at /home/lagrange/.local/share/greenlight
Your recovery mnemonic is: enact survey walk hen original permit tired betray when worry artefact ripple

If the user comes with his own mnemonic, either because it generated it with BIP85 "Determistic entropy", or dice, or he is recovering an existing node from scratch:

$ glcli node init --mnemonic "enact survey walk hen original permit tired betray when worry artefact ripple"
Local greenlight directory created at /home/lagrange/.local/share/greenlight
Secret seed derived from user provided mnemonic: enact survey walk hen original permit tired betray when worry artefact ripple