Add tests and syntax cleanup

* Adds serverspec tests for postfix.
* Switches from using `chomp` to remove newlines from output to using `split`. `chomp` was leaving trailing whitespace if it existed in output from `postconf`.
* Checks to make sure that `postconf` output line has a value before doing assignment. Otherwise the output of `foo = ` would cause a `nil` exception (Ubuntu seems to have a lot of these).
* Disables ipv6 for Cent tests because lots of host not found errors trying to connect to repos via ipv6.
* Adds postfix recipe (postfix is installed by default on Cent, but not Ubuntu).