simp_options::firewall doesn't allow users to disable host based firewalls

If a user sets `simp_options::firewall: false` host based firewall rules are not disabled on any host.

This seems to spawn from these two lines of code:

• https://github.com/simp/pupmod-simp-iptables/blob/master/manifests/init.pp#L83

• https://github.com/simp/pupmod-simp-iptables/blob/master/manifests/init.pp#L97

The `iptables::enable` parameter does an explicit lookup on the firewall global but the actual logic in the iptables module (the second link) only honors a string that matches 'ignore' to not enforce firewall rules. Not only is this confusing but it's worth noting that the firewall global is strongly typed as a boolean so you can't even set it to the string 'ignore'.