--reap flag is not added to iptables command

[simon-martin]

In lib/puppet/provider/firewall/iptables.rb we test on boolean flags when building iptables args:

    # If socket is true then do not add the value as -m socket is standalone
    if known_booleans.include?(res) then
      if resource[res] == :true then
        resource_value = nil
      else
        # If the property is not :true then we don't want to add the value
        # to the args list
        next
      end
    end

This evaluates to false on the reap flag in a definition like this:
firewall { '001 rate limit ssh attempts':
port => [22],
proto => tcp,
tcp_flags => "FIN,SYN,RST,ACK SYN",
recent => 'rcheck',
rsource => true,
rname => 'ssh-syn4',
rseconds => 30,
rhitcount => 3,
reap => true,
jump => drop,
}

This is because the value is not defined as a string, so the reap flag is not added to the args. This patch defines reap as a string true or false to match others like rsource.