Updated the StackdriverWriter output plugin to support GCE

Response to a user request to support auto-detection of instance IDs on Google Compute Engine (GCE).  They have a very similar metadata service to the AWS one that was supported, so I made the lookup a bit more flexible and split the config out as parameters.  Now supports detectInstance="AWS" or detectInstance="GCE" and pulls the local instance ID on those cloud providers.