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

Acceptance test output:

$ make testacc TEST=./aws/ TESTARGS='-run=TestAccAWSInstance_rootInstanceStore'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws/ -v -count 1 -parallel 20 -run=TestAccAWSInstance_rootInstanceStore -timeout 120m
=== RUN   TestAccAWSInstance_rootInstanceStore
=== PAUSE TestAccAWSInstance_rootInstanceStore
=== CONT  TestAccAWSInstance_rootInstanceStore
--- PASS: TestAccAWSInstance_rootInstanceStore (129.72s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	129.765s

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