Check for iptables before trying to run iptables

Fixes #13410.
Having this in a __virtual__ means that it gets run every time the loader runs (which is a lot) and forces Salt to spin up an iptables process. (The LazyLoader solves some of this, but it's not in use for exec modules at present.) What do you think of moving this out of __virtual__ and into the only function where it is used?