Google Compute Engine metadata facts need endpoint change

Google Compute Engine's internal metadata service will be deprecating the 'v1beta1' endpoint sometime before end of calendar year 2019. The GCE resolver[1] should be updated to use the 'v1' endpoint instead.

Note that using the 'v1' endpoint also requires setting a custom User-Agent header that was not necessary in the old 'v1beta1' endpoint.  Using 'curl' in a simple bash script as an example, I would need to change,

curl http://metadata.google.internal/computeMetadata/v1beta1/instance/name

to this,

curl -H "Metadata-flavor: Google" http://metadata.google.internal/computeMetadata/v1/instance/name

 For more details about GCE metadata, please see https://cloud.google.com/compute/docs/storing-retrieving-metadata

[1] https://github.com/puppetlabs/facter/blob/master/lib/src/facts/resolvers/gce_resolver.cc#L243