builder/googlecompute: Selectively set default network

If a network is not specified, it should only be set to "default" if a
subnetwork is also not specified.

builder/googlecompute/config.go

@@ -82,7 +82,7 @@ func NewConfig(raws ...interface{}) (*Config, []string, error) {
 	var errs *packer.MultiError
 
 	// Set defaults.
-	if c.Network == "" {
+	if c.Network == "" && c.Subnetwork == "" {
 		c.Network = "default"
 	}