Change metadata url to FQDN

If instance has a different DNS/search domain then "metadata" alone won't work. The FQDN of "metadata.google.internal" is included in hosts file, and will work regardless of DNS configuration. Alternatively the IP 169.254.169.254 can be used.

builder/googlecompute/startup.go

@@ -15,7 +15,7 @@ const StartupScriptStatusNotDone string = "notdone"
 var StartupScriptLinux string = fmt.Sprintf(`#!/usr/bin/env bash
 echo "Packer startup script starting."
 RETVAL=0
-BASEMETADATAURL=http://metadata/computeMetadata/v1/instance/
+BASEMETADATAURL=http://metadata.google.internal/computeMetadata/v1/instance/
 
 GetMetadata () {
   echo "$(curl -f -H "Metadata-Flavor: Google" ${BASEMETADATAURL}/${1} 2> /dev/null)"