provider/aws: Provide the option to skip_destroy on aws_volume_attachment

[stack72]

When you want to attach and detach pre-existing EBS volumes to an
instance, we would do that as follows:

resource "aws_instance" "web" {
	ami = "ami-21f78e11"
  availability_zone = "us-west-2a"
	instance_type = "t1.micro"
	tags {
		Name = "HelloWorld"
	}
}

data "aws_ebs_volume" "ebs_volume" {
  filter {
  	name = "size"
  	values = ["${aws_ebs_volume.example.size}"]
  }
  filter {
  	name = "availability-zone"
  	values = ["${aws_ebs_volume.example.availability_zone}"]
  }
  filter {
  	name = "tag:Name"
  	values = ["TestVolume"]
  }
}

resource "aws_volume_attachment" "ebs_att" {
  device_name = "/dev/sdh"
	volume_id = "${data.aws_ebs_volume.ebs_volume.id}"
	instance_id = "${aws_instance.web.id}"
	skip_destroy = true
}

The issue here is that when we run a terraform destroy command, the volume tries to get detached from a running instance and can go into a non-responsive state. We would have to force_destroy the volume at that point and risk losing any data on it.

This PR introduces the idea of skip_destroy on a volume attachment. tl;dr:

We want the volume to be detached from the instane when the instance itself has been destroyed. This way the normal shut procedures will happen and protect the disk for attachment to another instance

Volume Attachment Tests:

% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSVolumeAttachment_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2016/11/02 00:47:27 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSVolumeAttachment_ -timeout 120m
=== RUN   TestAccAWSVolumeAttachment_basic
--- PASS: TestAccAWSVolumeAttachment_basic (133.49s)
=== RUN   TestAccAWSVolumeAttachment_skipDestroy
--- PASS: TestAccAWSVolumeAttachment_skipDestroy (119.64s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	253.158s

EBS Volume Tests:

% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSEBSVolume_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2016/11/02 01:00:18 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSEBSVolume_ -timeout 120m
=== RUN   TestAccAWSEBSVolume_importBasic
--- PASS: TestAccAWSEBSVolume_importBasic (26.38s)
=== RUN   TestAccAWSEBSVolume_basic
--- PASS: TestAccAWSEBSVolume_basic (26.86s)
=== RUN   TestAccAWSEBSVolume_NoIops
--- PASS: TestAccAWSEBSVolume_NoIops (27.89s)
=== RUN   TestAccAWSEBSVolume_withTags
--- PASS: TestAccAWSEBSVolume_withTags (26.88s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	108.032s

[jen20]

One minor doc change, otherwise LGTM!

[jen20]

"Set this to true if you do not wish to detach the volume from the instance to which it is attached at destroy time, and instead just remove the attachment from Terraform state. This is useful when destroying an instance which has volumes created by some other means attached."

[ghost]

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