Make shorewall_version fact fail gracefully if shorewall not installed
Fixing path to shorewall6
Make shorewall_version default to 0

Reverse the nesting of the Facter.add and the if blocks so that the version
variable's default value of 0 ends up being the fact value, and the fact always
exists.
Merge pull request #7 from inkblot/version-fact-graceful-failure

Also closes #6