Issue

close #4046

Description

• Added --default-subnets option
  • When subnets are not explicitly specified through annotations or other methods, the subnets specified in --default-subnets will be selected

Background

• When subnets are not explicitly specified through annotations, subnets are selected in lexicographical order within each availability zone, and the load balancer is created with those subnets. (doc)

• Due to this specification, if there are specific subnets where ELBs should be launched by default, annotations need to be added to all load balancers, which is cumbersome.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes

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 🌟

Manual tests

• Prerequisites

  • AZ a

    • subnet-1a-public
    • subnet-2a-public
    • subnet-1a-private
    • subnet-2a-private

  • AZ b

    • subnet-1b-public
    • subnet-2b-public
    • subnet-1b-private
    • subnet-2b-private

  • AZ c

    • subnet-1c-public
    • subnet-2c-public
    • subnet-1c-private
    • subnet-2c-private

  • Default selection (lexicographical order)

    • Internet facing ELB is created with subnet-1a-public, subnet-1b-public, subnet-1c-public
    • Internal ELB is created with subnet-1a-private, subnet-1b-private, subnet-1c-private

• Pattern 1

  • Specified subnet-2a-public,subnet-2b-public,subnet-2a-private in --default-subnets option
  • result:
    • Internet facing ELB is created with subnet-2a-public,subnet-2b-public,subnet-1c-public
    • Internal ELB is created with subnet-2a-private, subnet-1b-private, subnet-1c-private

• Pattern 2

  • Specified subnet-2a-public,subnet-1a-public in --default-subnets option
  • result:
    • Internet facing ELB is created with subnet-2a-public,subnet-1b-public,subnet-1c-public
    • Internal ELB is created with subnet-1a-private, subnet-1b-private, subnet-1c-private
    • (when two subnets from the same AZ are specified, the first one takes precedence)