Allow SSH and nginx to be more easily configured

This MR allows SSH and nginx to be more easily configured using gdk.yml or gdk.example.yml:

SSH

• sshd_enabled - Enable sshd to run in Procfile (default false)
• sshd_ip - The IP address sshd should listen on (default 127.0.0.1)
• sshd_port - The port sshd should listen on (default 2222)

We also now default to using the DB to validate SSH keys by default.

nginx

• nginx_enabled - Enable nginx to run in Procfile (default false)
• nginx_bin - The full path to the nginx binary (default find_executable!('nginx') || '/usr/sbin/nginx')
• nginx_ip - The IP address nginx should listen on (default 127.0.0.1)
• nginx_workhorse_port - The port nginx should connect to workhorse on (default 3333)
• nginx_https_port - The port nginx should listen on for HTTPS (default 3443)
• nginx_ssl_certificate_filename - The SSL cert filename nginx should look for (default localhost.crt)
• nginx_ssl_certificate_key_filename - The SSL key filename nginx should look for (default localhost.key)

⚠ This MR relies upon !782 (merged)