provider/aws: Stop aws_instance source_dest_check triggering an API call on each terraform run

[stack72]

Fixes #3550

The simple fix here was to check if the Resource was new (to set the
value the first time) then check it has changed each time

I was able to see from the TF log the following:

Config

resource "aws_vpc" "foo" {
    cidr_block = "10.10.0.0/16"
}

resource "aws_subnet" "foo" {
    cidr_block = "10.10.1.0/24"
    vpc_id = "${aws_vpc.foo.id}"
}

resource "aws_instance" "foo" {
    ami = "ami-4fccb37f"
    instance_type = "m1.small"
    subnet_id = "${aws_subnet.foo.id}"
    source_dest_check = false
        disable_api_termination = true
}

No longer caused any Modifying source_dest_check entries in the LOG

I'd like to leave this out of 0.7.2 so that we get a couple of acceptance test runs on this change!

[stack72]

Initial config gave this:

[stacko@Pauls-MacBook-Pro:~/Code/terraform-recreations/3550]
% TF_LOG=1 terraform apply 2>~/tf.log
aws_vpc.foo: Creating...
  cidr_block:                "" => "10.10.0.0/16"
  default_network_acl_id:    "" => "<computed>"
  default_security_group_id: "" => "<computed>"
  dhcp_options_id:           "" => "<computed>"
  enable_classiclink:        "" => "<computed>"
  enable_dns_hostnames:      "" => "<computed>"
  enable_dns_support:        "" => "<computed>"
  instance_tenancy:          "" => "<computed>"
  main_route_table_id:       "" => "<computed>"
aws_vpc.foo: Still creating... (10s elapsed)
aws_vpc.foo: Creation complete
aws_subnet.foo: Creating...
  availability_zone:       "" => "<computed>"
  cidr_block:              "" => "10.10.1.0/24"
  map_public_ip_on_launch: "" => "false"
  vpc_id:                  "" => "vpc-ccfacea8"
aws_subnet.foo: Creation complete
aws_instance.foo: Creating...
  ami:                      "" => "ami-4fccb37f"
  availability_zone:        "" => "<computed>"
  ebs_block_device.#:       "" => "<computed>"
  ephemeral_block_device.#: "" => "<computed>"
  instance_state:           "" => "<computed>"
  instance_type:            "" => "m1.small"
  key_name:                 "" => "<computed>"
  network_interface_id:     "" => "<computed>"
  placement_group:          "" => "<computed>"
  private_dns:              "" => "<computed>"
  private_ip:               "" => "<computed>"
  public_dns:               "" => "<computed>"
  public_ip:                "" => "<computed>"
  root_block_device.#:      "" => "<computed>"
  security_groups.#:        "" => "<computed>"
  source_dest_check:        "" => "false"
  subnet_id:                "" => "subnet-cade6c92"
  tenancy:                  "" => "<computed>"
  vpc_security_group_ids.#: "" => "<computed>"
aws_instance.foo: Still creating... (10s elapsed)
aws_instance.foo: Still creating... (20s elapsed)
aws_instance.foo: Still creating... (30s elapsed)
aws_instance.foo: Still creating... (40s elapsed)
aws_instance.foo: Creation complete

Apply complete! Resources: 3 added, 0 changed, 0 destroyed.

Adding disable_api_termination = true would trigger an update and the old code would show that source_dest_check would be modified:

[stacko@Pauls-MacBook-Pro:~/Code/terraform-recreations/3550]
% TF_LOG=1 terraform apply 2>~/tf.log                                                                                                                                                                   
aws_vpc.foo: Refreshing state... (ID: vpc-ccfacea8)
aws_subnet.foo: Refreshing state... (ID: subnet-cade6c92)
aws_instance.foo: Refreshing state... (ID: i-b8e3bb65)
aws_instance.foo: Modifying...
  disable_api_termination: "false" => "true"
aws_instance.foo: Modifications complete

Apply complete! Resources: 0 added, 1 changed, 0 destroyed.

The state of your infrastructure has been saved to the path
below. This state is required to modify and destroy your
infrastructure, so keep it safe. To inspect the complete state
use the `terraform show` command.

State path: terraform.tfstate

[mitchellh]

Oh well done. LGTM.

[ghost]

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