(#17106) ip6tables provider is still calling the iptables and iptables-save commands

This resource was creating the rule in the ***iptables*** list instead of the ***ip6tables*** one:
```puppet
  firewall { '100 allow glusterd on ipv6':
    proto   => 'tcp',
    dport   => '24007',
    action  => accept,
    provider => 'ip6tables',
  }
```
This patch adds support to utilize the commands defined for the specific provider being used.