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

Acceptance test output:

$ make testacc TEST=./aws/ TESTARGS='-run=TestAccAWSInstance_RootBlockDevice_KmsKeyArn'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws/ -v -count 1 -parallel 20 -run=TestAccAWSInstance_RootBlockDevice_KmsKeyArn -timeout 120m
=== RUN   TestAccAWSInstance_RootBlockDevice_KmsKeyArn
=== PAUSE TestAccAWSInstance_RootBlockDevice_KmsKeyArn
=== CONT  TestAccAWSInstance_RootBlockDevice_KmsKeyArn
--- PASS: TestAccAWSInstance_RootBlockDevice_KmsKeyArn (121.92s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	121.975s

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