allow ssh_host to override the host ip gathered from the instance for GCP builder

builder/googlecompute/ssh.go

@@ -5,6 +5,10 @@ import (
 )
 
 func commHost(state multistep.StateBag) (string, error) {
+	config := state.Get("config").(*Config)
+	if config.Comm.SSHHost != "" {
+		return config.Comm.SSHHost, nil
+	}
 	ipAddress := state.Get("instance_ip").(string)
 	return ipAddress, nil
 }