Add diff to mx_l3_firewall module

[szeestraten]

Hi, it would be nice to have diff functionality in the mx_l3_firewall module. I saw the previous issue #37 regarding diffs so I hope it's OK to put in a request here.

[kbreit]

I was looking into this and it should be possible. The way this module works does complicate it but let me look into it.

[kbreit]

I've been working on this feature tonight and the module logic is awful because the default rule significantly complicates things. I may refactor the module altogether and add it then.

[szeestraten]

@kbreit thank you very much for taking a look. The default allow any rule I presume (that caught me out a couple of times)? Just a heads up that I'll most likely touch this module in #316. If it's ok for you, I'll just work from the current implementation there.

[kbreit]

Good point. That's fine go ahead and work on the module.