Add support for PREROUTING and POSTROUTING chains

If you need to define a rule into PREROUTING or POSTROUTING chain, e.g.:

    iptables::rule { 'redirect-80-to-8000':
        command => '-A',
        chain => 'PREROUTING',
        target => 'REDIRECT',
        rule => '-p tcp --dport 80 --to-port 8000'
    }

it will end up in this error message:

    Error: No matching value for selector param 'PREROUTING'
    at /tmp/vagrant-puppet/modules-0/iptables/manifests/rule.pp:72 on node web-server