Gas, hybrid, plug-in, fully electric; that’s the standard progression a vehicle takes on its path to greener pastures. Take this Cayenne, for example. Introduced 15 years ago, Porsche added a hybrid variant in 2011 and upgraded its capabilities with a plug 4 years later. Over that time, I’ve tested every Cayenne model and this week it’s time to see what this plug-in, or E-Hybrid, has to offer in 2017.Commonly referred …