Support Linux kernel 2.6 without xt_comment module for iptables

Sample error:

```
debug: Puppet::Type::Firewall::ProviderIptables: Executing '/sbin/iptables-save'
debug: Firewall[000 accept all icmp](provider=iptables): Inserting rule 000 accept all icmp
debug: Firewall[000 accept all icmp](provider=iptables): [insert_order]
debug: Puppet::Type::Firewall::ProviderIptables: [instances]
debug: Puppet::Type::Firewall::ProviderIptables: Executing '/sbin/iptables-save'
debug: Firewall[000 accept all icmp](provider=iptables): Current resource: Puppet::Type::Firewall                                                                                             
debug: Puppet::Type::Firewall::ProviderIptables: Executing '/sbin/iptables -I INPUT 1 -t filter -p icmp -m comment --comment 000 accept all icmp -j ACCEPT'                                   
err: /Firewall[000 accept all icmp]/ensure: change from absent to present failed: Execution of '/sbin/iptables -I INPUT 1 -t filter -p icmp -m comment --comment 000 accept all icmp -j ACCEPT' returned 1: iptables: No chain/target/match by that name.
```

BuyVM.net OpenVZ VPS uses kernel 2.6 without xt_comment module.

Original issue: https://github.com/soluvas/puppet-firewall/issues/1