r/aws_instance: Remove hardcoded AMI ID from 'TestAccAWSInstance_GP2IopsDevice'.
r/aws_instance: Remove hardcoded AMI ID from 'TestAccAWSInstance_GP2IopsDevice'.

Acceptance test output:

$ make testacc TEST=./aws/ TESTARGS='-run=TestAccAWSInstance_GP2IopsDevice'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws/ -v -count 1 -parallel 20 -run=TestAccAWSInstance_GP2IopsDevice -timeout 120m
=== RUN   TestAccAWSInstance_GP2IopsDevice
=== PAUSE TestAccAWSInstance_GP2IopsDevice
=== CONT  TestAccAWSInstance_GP2IopsDevice
--- PASS: TestAccAWSInstance_GP2IopsDevice (100.62s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	100.710s

terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws