Allow use of Default Application Credentials for GCE
Allow use of Default Application Credentials for GCE

Also allows project and zone to be specified by the config file, rather
than automatically overwritten. This allows local testing against the
GCE API without having to run on a GCE instance. The default credentials
take priority over the token source, but less priority than a token
source in the config.

Also, if the node template cannot be located, don't panic, just report
it.

mtcode/autoscaler,kgrygiel/autoscaler,kubernetes/autoscaler,KarolKraskiewicz/autoscaler,mmerrill3/autoscaler,KarolKraskiewicz/autoscaler,kubernetes/autoscaler,bskiba/autoscaler,mmerrill3/autoscaler,bskiba/autoscaler,aleksandra-malinowska/autoscaler,mtcode/autoscaler,kubernetes/autoscaler,bskiba/autoscaler,mmerrill3/autoscaler,mmerrill3/autoscaler,aleksandra-malinowska/autoscaler,KarolKraskiewicz/autoscaler,kgrygiel/autoscaler,mtcode/autoscaler,kgrygiel/autoscaler,kubernetes/autoscaler,aleksandra-malinowska/autoscaler