[monitoring] Allow collecting ARP table information in DeviceData.data

[nemesifier]

Like #33, but with ARP table information.

Incoming ARP table data:

{
    'type': 'DeviceMonitoring',
    'arp_table': [
        {
            'ip_address': '192.168.66.197',  # ipv4 or ipv6
            'mac_address': 'f2:f1:3e:56:d2:66',  # mac address
            'interface': 'br-lan',  # string
            'type': '0x1',  # string
            'flags': '0x2',  # string
            'mask': '*',  # string
        }
    }
}

The schema in openwisp_monitoring.device.schema will have to be updated accordingly.

Once the data is saved and validated correctly (a few tests will need to ensure bogus arp info will be rejected) we will have to ensure the arp table info is shown in the device status tab (type, flags and mask can be ignored for the moment):
https://github.com/openwisp/openwisp-monitoring/blob/master/openwisp_monitoring/device/templates/admin/config/device/change_form.html#L66-L124

If the arp table info is not available, do not mention this in the admin UI.

Will need openwisp/openwrt-openwisp-monitoring#3 to be implemented in order to send the ARP information from the device.

[PabloCastellano]

@nemesisdesign I'm close to finishing ARP table data collection. What kind of tests are appropriate to include? As I understand, JSON-schema validates the data received from the device but the validation is very simple (e.g. it does not check if the string received in the ip_address field is an IP address indeed).

[nemesifier]

@PabloCastellano it should be possible to specify that it should be an ip, don't worry about it now, let's do a few iterations