ip: ipv4.forwarding deserialize from string also

Be able to use "true" in place of true in ipv4.forwarding:

```
interfaces:
  - name: eth1
    type: ethernet
    state: up
    ipv4:
      address:
      - ip: 192.0.2.1
        prefix-length: 24
      dhcp: false
      enabled: true
      forwarding: "true"
```

Resolves: https://issues.redhat.com/browse/RHEL-153618

Signed-off-by: Rahul Rajesh <rajeshrah22@gmail.com>