Only override vpc endpoint type on govcloud if empty.

Once govcloud implements multiple vpc endpoint types, we should stop
forcing the endpoint type to gateway.

h/t @wjwoodson

aws/resource_aws_vpc_endpoint.go

@@ -347,7 +347,7 @@ func vpcEndpointAttributes(d *schema.ResourceData, vpce *ec2.VpcEndpoint, meta i
 
 	serviceName := aws.StringValue(vpce.ServiceName)
 	d.Set("service_name", serviceName)
-	if meta.(*AWSClient).IsGovCloud() {
+	if meta.(*AWSClient).IsGovCloud() && aws.StringValue(vpce.VpcEndpointType) == "" {
 		d.Set("vpc_endpoint_type", ec2.VpcEndpointTypeGateway)
 	} else {
 		d.Set("vpc_endpoint_type", vpce.VpcEndpointType)