Document answer_file install options

[dn0]

Should be places on the USB image into private/answers.json.

Example:

{
    "config_console": "vga",
    "skip_instructions": true,
    "simple_headers": true,
    "skip_final_confirm": true,
    "accept_eula": false,
    "datacenter_name": "XXX",
    "advanced_install": false,
    "add_nictags": false,
    "admin_ip": "XXX",
    "admin_netmask": "XXX",
    "admin_gateway": "XXX",
    "skip_ntp_check": false,
    "headnode_default_gateway": "XXX",
    "dns_resolver1": "<default>",
    "dns_resolver2": "<default>",
    "dns_domain": "<default>",
    "dns_search": "<default>",
    "ntp_host": "<default>",
    "hostname": "<default>",
    "root_password": "root",
    "admin_email": "root@example.com",
    "mgmt_admin_ip": "XXX",
    "esdc_install_password": "XXX",
    "disk_layout": "<default>",
    "install_to_hdd": false
}