refactor: warn with few AZs and separate subnet iteration

[dekobon]

Proposed changes

Pulumi will now log a warning when a VPC is created when there
are two or less availability zones available. Additionally, this
change breaks apart the logic for subnet iteration from the
list of availability zones in order to make increasing the
number of subnets easier.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have written my commit messages in the Conventional Commits format.
• I have read the CONTRIBUTING doc
• I have added tests (when possible) that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto master
• I will ensure my PR is targeting the master branch and pulling from my branch from my own fork

[qdzlug]

Looks good - this is a helpful change.