fix error when primary attribute is specified in multiple regions

[andy-kimball]

Previously, the TF provider raised an error when the primary attribute was specified in multiple regions, e.g.:

regions = [ { name = "eu-west-1" primary = true }, { name = "eu-west2" primary = false }, { name = "ap-south-1" primary = false }, ]

This PR fixes that by adding some custom validation logic rather than using the built-in config validators.

Fixes: CC-27160

Commit checklist

• [X] Changelog
• [X] Doc gen (make generate)
• [X] Integration test(s)
• [X] Acceptance test(s)
• [X] Example(s)

[andy-kimball]

TFTR