Fix handling of vpc_zone_identifier for autoscaling groups in EC2 classic

[joshuaspence]

Fixes hashicorp/terraform#6417.

> make testacc TESTARGS='-run=TestAccAWSAutoScalingGroup_classicVpcZoneIdentifier'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccAWSAutoScalingGroup_classicVpcZoneIdentifier -timeout 120m
?       github.com/terraform-providers/terraform-provider-aws   [no test files]
=== RUN   TestAccAWSAutoScalingGroup_classicVpcZoneIdentifier
--- PASS: TestAccAWSAutoScalingGroup_classicVpcZoneIdentifier (90.85s)
PASS
ok      github.com/terraform-providers/terraform-provider-aws/aws   90.878s

[radeksimko]

Good catch and thanks for the acceptance test 👍 This LGTM.

[radeksimko]

btw. I think this isn't just EC2 Classic specific problem, it may affect VPC accounts too. AFAIK the ASG will just use subnets from the default VPC if none are defined.

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!