fix(coreos-base/oem-gce): Silence bogus /etc/hosts error

On first boot /etc/hosts doesn't exist yet, causing an error to be
logged. This is a bit misleading
fix(coreos-base/oem-gce): Robustify the GCE user-data script.

 - Provide some logging on what actually happened.
 - Tolerate empty user-data, just no-op.
 - Apply both project and instance data.
Merge pull request #542 from marineam/gcefix

GCE fixes