Allow configurable path to `service`

Hello! First, thanks for this awesomely configurable cookbook. It has made it possible for me to find the joy of smartstack without needing to configure barely anything to begin with.

I know this cookbook is intended for Ubuntu audiences only, but we're looking at using it on CentOS. Only one snag to be found (so far?): the path to the service executable is different so SmartStack cannot restart haproxy.

This PR is a quick hack allowing a different path based on node['platform'], and for other platforms to be able to configure it via an attribute. I'd be happy to rewrite a different way if you think of a better way.

Also, is there a convenient way to run the test suite against this? I did some minimal verification locally but some continuous integration here could go a long way.

Thanks!