r/aws_instance: Remove hardcoded AMI ID from 'TestAccAWSInstance_userDataBase64'.
r/aws_instance: Remove hardcoded AMI ID from 'TestAccAWSInstance_userDataBase64'.

Acceptance test output:

$ make testacc TEST=./aws/ TESTARGS='-run=TestAccAWSInstance_userDataBase64'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws/ -v -count 1 -parallel 20 -run=TestAccAWSInstance_userDataBase64 -timeout 120m
=== RUN   TestAccAWSInstance_userDataBase64
=== PAUSE TestAccAWSInstance_userDataBase64
=== CONT  TestAccAWSInstance_userDataBase64
--- PASS: TestAccAWSInstance_userDataBase64 (136.34s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	136.400s

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