provider/aws: Make all fields in ecs_task_definition ForceNew

[radeksimko]

Fixes #2694

You might be thinking this will cause troubles since we'd be deleting task definitions that are in use by existing ECS service, but due to the nature of TDs & deregistration it shouldn't be any problem:

http://docs.aws.amazon.com/AmazonECS/latest/developerguide/deregister-task-definition.html

When you deregister a task definition, it is immediately marked as INACTIVE. Existing tasks and services that reference an INACTIVE task definition continue to run without disruption, and existing services that reference an INACTIVE task definition can still scale up or down by modifying the service's desired count.

Test plan

$ make testacc TEST=./builtin/providers/aws TESTARGS='-run=EcsTaskDefinition' 2>~/tf.log

go generate ./...
TF_ACC=1 go test ./builtin/providers/aws -v -run=EcsTaskDefinition -timeout 90m
=== RUN   TestAccAWSEcsTaskDefinition_basic
--- PASS: TestAccAWSEcsTaskDefinition_basic (16.32s)
=== RUN   TestAccAWSEcsTaskDefinition_withScratchVolume
--- PASS: TestAccAWSEcsTaskDefinition_withScratchVolume (9.39s)
=== RUN   TestAccAWSEcsTaskDefinition_withEcsService
--- PASS: TestAccAWSEcsTaskDefinition_withEcsService (105.88s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    131.603s

[catsby]

LGTM. The 'a way to trigger "recomputation" of Computed fields under certain conditions' point is a good one, the details of which may not be that straight forward to implement though. Will discuss.

[ghost]

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