(#15038) add gre protocol to list of acceptable protocols

[jasonhancock]

Add gre protocol to list of acceptable protocols. A resource such as this:

firewall { '001 accept gre':
  proto   => 'gre',
  action  => 'accept',
}

Was previously thowing an error:

Failed to apply catalog: Parameter proto failed: Invalid value "gre". Valid values are tcp, udp, icmp, ipv6-icmp, esp, ah, vrrp, igmp, ipencap, all.  at /etc/puppet/somemanifest.pp:27

With my patch applied, the rule gets created correctly:

# iptables -L | grep gre
ACCEPT     gre  --  anywhere             anywhere            /* 001 accept gre */