Allow overriding of prefix separator in self-managed node groups

[james-menzies]

Description

Add a prefix_separator variable into the self_managed_node_groups variable schema, and use it to override the hard-coded '-' separator when providing name_prefix values to underlying AWS resources. This is functionally identical to the prefix_separator variable on the top level-variable definition.

Motivation and Context

I've found that when upgrading from v17 when using a self-managed node group, that the aws_launch_template resource can take a while to replace (around 7 minutes) and in that time, the cluster is not able to receive any incoming traffic.

In the v18 upgrade guide, the strategy around using the prefix_separator to replace '-' with '' works well with respect to ensuring that the EKS cluster itself can be updated in-place, however the same strategy cannot be applied to the AWS Launch template resource due to the hard coded hyphen separator.

Breaking Changes

No breaking changes. I did consider propagating the prefix_separator directly from the top-level variable schema, however decided against that as it would potentially create breaking behaviour for existing users.

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects

• I have executed pre-commit run -a on my pull request

For testing:

• I first applied the self-managed node groups example from the master branch.
• I then changed to the head of this branch and re-ran terraform plan.
• There were no changes (expected).
• Then I added an explicit prefix_separator of -.
• Again, no changes (expected).
• Then I changed the separator to an empty string.
• All affected resources that use the prefix separator needed to be replaced (expected).

I didn't see any use of straying from using the default names in the examples directory, so I didn't add anything to it, but am happy to update it if desired.

[james-menzies]

I opted for this strategy rather than coalesce so that an empty string is not clobbered by the default.

[bryantbiggs]

Thank you, but unfortunately this is not a change that we are looking to carry forward

[james-menzies]

I replaced all instances where I felt the separator was hard-coded in this file for consistency, but am happy to take some out if appropriate.