feat(elasticache-redis): allow specifying environment for dns-delegated component

[marsadle]

what

Add dns_gbl_delegated_environment_name (used throughout components) to specify environment of dns-delegated component.

why

This change is necessary when deploying identical environments in multiple regions and using regional dns-delegated components rather than a single global one.

[Nuru]

@marsadle Why are you using regional dns-delegated components? DNS is not regional in any way, and our architecture is set up for you to use a single dns-delegated component per account.

[marsadle]

Your dns-delegated component actually describes, that it can be deployed regionally.
Not all components are set up to do so, e.g. aurora-postgres which has no way to point to a regional endpoint. We need postgres-writer.euw1.our-infrastructure.cloud and postgres-writer.use1.our-infrastructure.cloud which are entirely separate environments. The component only provisions postgres-writer.our-infrastructure.cloud when provisioned with a global dns-delegated zone.

This is a change that is non-intrusive and has been made across components (e.g. aurora-postgres), which is much easier to do than to alter components into setting custom DNS names.

[Nuru]

I apologize for the misleading documentation (will fix). dns-delegated should only be deployed globally. See Why should the dns-delegated component be deployed globally rather than regionally?.

I am rejecting this PR as an anti-pattern.

In your case, you can set the regional designator in var.cluster_dns_name_part and var.reader_dns_name_part. Set var.cluster_dns_name_part to writer.euw1 or writer.use1 as appropriate (and likewise for reader), and continue to use a single global dns-delegated component.

While one could argue that the module could have better support for your use case, this PR is not it.

@marsadle Please know that we are grateful for your effort to improve the module, and in this case you have inspired better documentation, even though this PR itself is not being accepted. Thank you.