Inventory: advanced usage statistics

[stephdl]

Add more statistics to the inventory

the json object output

json sample

    "netifyd": {
      "enabled": false
    },
    "snmp": {
      "enabled": true
    },
    "ddns": {
      "enabled": true
    },
    "nathelpers": {
      "count": 2,
      "enabled": true
    },
    "certificates_info": {
      "custom_certificates": {
        "count": 1
      },
      "acme_certificates": {
        "count": 1,
        "issued": 0,
        "pending": 1
      }
    },
    "database_stats": {
      "main": {
        "users": 2
      }
    },
    "dhcp_server": {
      "count": 1,
      "static_leases": 1,
      "dynamic_leases": 0,
      "dns_records_count": 2,
      "dns_forwarder_enabled": true
    },
    "firewall_stats": {
      "firewall": {
        "port_forward": 1,
        "nat": {
          "masquerade": 1,
          "snat": 1,
          "accept": 1
        },
        "netmap": {
          "source": 1,
          "destination": 1
        },
        "rules": {
          "forward": 1,
          "input": 9,
          "output": 1
        }
      },
      "objects": {
        "domains": 1,
        "hosts": 1,
        "port_forward": {
          "allowed_from": 0,
          "destination_to": 1
        },
        "mwan_rules": 1,
        "rules": {
          "forward": 1,
          "input": 1,
          "output": 1
        }
      }
    },
    "hotspot": {
      "enabled": true,
      "server": "https://my.nethspot.com/api",
      "interface": "eth3"
    },
    "network": {
      "zones": [
        {
          "name": "lan",
          "ipv4": 1,
          "ipv6": 0
        },
        {
          "name": "wan",
          "ipv4": 1,
          "ipv6": 0
        },
        {
          "name": "rwopenvpn",
          "ipv4": 0,
          "ipv6": 0
        },
        {
          "name": "openvpn",
          "ipv4": 1,
          "ipv6": 0
        },
        {
          "name": "hotspot",
          "ipv4": 0,
          "ipv6": 0
        },
        {
          "name": "blue",
          "ipv4": 2,
          "ipv6": 0
        }
      ],
      "interface_counts": {
        "vlans": 1,
        "bridges": 1,
        "bonds": 1
      },
      "zone_network_counts": {
        "lan": 1,
        "wan": 1,
        "rwopenvpn": 1,
        "openvpn": 1,
        "hotspot": 1,
        "blue": 2
      },
      "route_info": {
        "count_ipv6_route": 2,
        "count_ipv4_route": 2
      }
    },

    "openvpn_rw": {
      "enabled": 1,
      "server": 1,
      "instances": [
        {
          "section": "ns_roadwarrior1",
          "authentication": "certificate",
          "user_database": "main",
          "mode": "tun"
        }
      ]
    },
    "openvpn_tun": {
      "client": 0,
      "server": 1,
      "tunnels": [
        {
          "section": "ns_openvpn1",
          "mode": "tun"
        }
      ]
    },

    "qos": {
      "count": 1,
      "rules": [
        {
          "enabled": true,
          "upload": 9,
          "download": 10
        }
      ]
    },

NethServer/nethsecurity#790