Add missing set_mark in the ip6tables provider

I was trying to port some of my existing ip6tables rules from custom templates to the firewall module, and I was getting this error for some IPVS related mark rules : 
```
Error: Execution of '/sbin/ip6tables -I PREROUTING 1 -t mangle -d 2001:ffff:4020:1010::1/128 -p tcp -m multiport --dports 443 -m comment --comment 040 ip2 https fwmark 4 -j MARK' returned 2: ip6tables v1.4.7: MARK: One of the --set-xmark, --{and,or,xor,set}-mark options is required
Try `ip6tables -h' or 'ip6tables --help' for more information.
```
I had a look at the provider, and the set_mark parameter seemed to be missing, so I just copied over the few relevant lined from the main IPv4 provider and it now works.

This is one of the rules inside my manifest : 
```puppet
firewall { '040 ip2 https fwmark 4':
  destination => '2001:ffff:4020:1010::1',
  proto       => 'tcp',
  dport       => '443',
  table       => 'mangle',
  chain       => 'PREROUTING',
  jump        => 'MARK',
  set_mark    => '4',
  provider    => 'ip6tables',
}
```