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

Acceptance test output:

$ make testacc TEST=./aws/ TESTARGS='-run=TestAccAWSInstance_EbsBlockDevice_KmsKeyArn'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws/ -v -count 1 -parallel 20 -run=TestAccAWSInstance_EbsBlockDevice_KmsKeyArn -timeout 120m
=== RUN   TestAccAWSInstance_EbsBlockDevice_KmsKeyArn
=== PAUSE TestAccAWSInstance_EbsBlockDevice_KmsKeyArn
=== CONT  TestAccAWSInstance_EbsBlockDevice_KmsKeyArn
--- PASS: TestAccAWSInstance_EbsBlockDevice_KmsKeyArn (114.53s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	114.602s

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