iptables backend was requested but configuration contains nftables-specific options [conditionsV4]

Hello @danwinship 

In #935, `detectBackendOfConditions` assumes all iptables rules start with `-`, mine start with `!`
My previously working config:
```
{
  "type": "portmap",
  "capabilities": {"portMappings": true},
  "externalSetMarkChain": "KUBE-MARK-MASQ",
  "conditionsV4": ["!", "-d", "127.0.0.0/8", "-m", "iprange", "!", "--dst-range", "198.19.254.254"]
},
```