0.33.0: fix_task_exec_role_arn: Fix problem where it still tries to create aw…

Change-Id: Ib507b196870340011e8afdfc1f381ae735ba9099

what

• The previous PR (#63) was incomplete and gives the following error:

Error: "name" must match [\w+=,.@-]

  on .terraform/modules/ecs_alb_service_task/terraform-aws-ecs-alb-service-task-0.32.0/main.tf line 195, in resource "aws_iam_role_policy" "ecs_exec":
 195: resource "aws_iam_role_policy" "ecs_exec" {

• The fix ensures that aws_iam_role_policy and aws_iam_policy_document are not created if task_exec_role_arn is passed in.

why

• Fix the bug.