Subnets availability validation should use AZs resolved by EC2::DescribeSubnets call

[TiberiuGC]

Description

The validation in question was using the subnet keys found in the config file. However, subnet key doesn't necessarily represent the subnet's AZ, but can be a user given alias. Instead, the validation shall use the subnet AZ fetched from EC2::DescribeSubnets call.

Fixes #7785

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the userdocs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes
• (Core team) Added labels for change area (e.g. area/nodegroup) and kind (e.g. kind/improvement)

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟