COOK-3367 Pass through more unicorn parameters

There are several configuration parameters supported by the unicorn cookbook that aren't passed through by the rails subresource of the application_ruby cookbook: at the moment, I want to specify both before_fork (supported) and after_fork (not), but there are several other useful ones.

Note that this allows overriding of the entire 'listen' parameter if port isn't enough; all the rest are simple.