provider/aws: Set aws_instance volume_tags to be Computed (#14007)

Fixes: #14003

When an EBS volume was created and tags were specified on that resource
and NOT the aws_instance it was attached to, the tags would be removed
on subsequent Terraform runs.

We need to set volume_tags to be Computed to allow for changes to EBS
volumes not created as part of the instance but that are attached to the
instance

```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSInstance_volumeTagsComputed'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/27 07:33:36 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSInstance_volumeTagsComputed -timeout 120m
=== RUN   TestAccAWSInstance_volumeTagsComputed
--- PASS: TestAccAWSInstance_volumeTagsComputed (151.37s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	151.411s
```

builtin/providers/aws/resource_aws_instance.go

@@ -234,7 +234,7 @@ func resourceAwsInstance() *schema.Resource {
 
 			"tags": tagsSchema(),
 
-			"volume_tags": tagsSchema(),
+			"volume_tags": tagsSchemaComputed(),
 
 			"block_device": {
 				Type:     schema.TypeMap,

builtin/providers/aws/resource_aws_instance_test.go

@@ -678,6 +678,25 @@ func TestAccAWSInstance_volumeTags(t *testing.T) {
 	})
 }
 
+func TestAccAWSInstance_volumeTagsComputed(t *testing.T) {
+	var v ec2.Instance
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckInstanceDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccCheckInstanceConfigWithAttachedVolume,
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckInstanceExists("aws_instance.foo", &v),
+				),
+				ExpectNonEmptyPlan: false,
+			},
+		},
+	})
+}
+
 func TestAccAWSInstance_instanceProfileChange(t *testing.T) {
 	var v ec2.Instance
 	rName := acctest.RandString(5)
@@ -1382,6 +1401,69 @@ resource "aws_instance" "foo" {
 }
 `
 
+const testAccCheckInstanceConfigWithAttachedVolume = `
+data "aws_ami" "debian_jessie_latest" {
+  most_recent = true
+
+  filter {
+    name   = "name"
+    values = ["debian-jessie-*"]
+  }
+
+  filter {
+    name   = "virtualization-type"
+    values = ["hvm"]
+  }
+
+  filter {
+    name   = "architecture"
+    values = ["x86_64"]
+  }
+
+  filter {
+    name   = "root-device-type"
+    values = ["ebs"]
+  }
+
+  owners = ["379101102735"] # Debian
+}
+
+resource "aws_instance" "foo" {
+  ami                         = "${data.aws_ami.debian_jessie_latest.id}"
+  associate_public_ip_address = true
+  count                       = 1
+  instance_type               = "t2.medium"
+
+  root_block_device {
+    volume_size           = "10"
+    volume_type           = "standard"
+    delete_on_termination = true
+  }
+
+  tags {
+    Name    = "test-terraform"
+  }
+}
+
+resource "aws_ebs_volume" "test" {
+  depends_on        = ["aws_instance.foo"]
+  availability_zone = "${aws_instance.foo.availability_zone}"
+  type       = "gp2"
+  size              = "10"
+
+  tags {
+    Name = "test-terraform"
+  }
+}
+
+resource "aws_volume_attachment" "test" {
+  depends_on  = ["aws_ebs_volume.test"]
+  device_name = "/dev/xvdg"
+  volume_id   = "${aws_ebs_volume.test.id}"
+  instance_id = "${aws_instance.foo.id}"
+}
+`
+
 const testAccCheckInstanceConfigNoVolumeTags = `
 resource "aws_instance" "foo" {
 	ami = "ami-55a7ea65"