Validate ipv4 address input

[bastelfreak]

We can insert a DHCP entry with a broken gateway:

curl -u admin:penis --data 'ip_address=178.63.176.36&mac=52:54:00:cf:5a:57&gateway=178.63.176.33=&networkmask=255.255.255.224' http://virtapi,de:5000/v1/dhcp

{
    "additional_statements": {},
    "dhcp_hostname": "virtapi.de",
    "gateway": "178.63.176.33=",
    "ip_address": "178.63.176.40",
    "mac": "52:54:00:cf:5a:58",
    "networkmask": "255.255.255.224"
}

this should be validated