Turn on Elasticsearch logging by default for GCE platform

As requested by @brendandburns this PR turns on by default Fluentd logging to Elasticsearch on GCE.
If logging to Google Cloud platform is requested then this takes precedent and Elasticsearch logging is suppressed.
All logging can be turned off by ensuring FLUENTD_ELASTICSEARCH is set if "false" (or any value other than "true") and by not setting FLUENTD_GCP or making sure it has a value other than "true".