Add support for SSL Certificate provisioning via ZeroSSL

[gkrizek]

This PR Adds support for automatically provisioning SSL Certificates for Thunderhub from ZeroSSL.

Configuration

This adds three new fields to the .env file:

 PUBLIC_URL='app.example.com' # Publicly reachable domain name
 SSL_PORT=8080                # Port to serve the HTTP Validation server from. If not port 80 it will need to be proxied to port 80.
 SSL_SAVE=true                # Whether to save the provisioned SSL Certificate to disk to reuse after restart.

Also, when using the secure server, you must set NODE_ENV=production and also set your ZeroSSL API key with ZEROSSL_API_KEY=abc123

Start

$ npm run start:secure

Example Startup

$ NODE_ENV=production npm run start:secure

> thunderhub@0.11.0 start:secure /root/thunderhub
> node secure-server.js

Loaded env from /root/thunderhub/.env
=> [ssl] Generated CSR
=> [ssl] Requested certificate
=> [ssl] validation server started at http://0.0.0.0:80
=> [ssl] waiting for certificate to be issued
=> [ssl] checking certificate again...
=> [ssl] checking certificate again...
=> [ssl] checking certificate again...
=> [ssl] certificate was issued
=> [ssl] validation server stopped.

ready - started server on http://localhost:3000