add support for eksctl utils update-legacy-subnet-settings with a cfg file

[aclevername]

Description

#1126

Before:

eksctl utils update-legacy-subnet-settings -f cluster.yaml
Error: unknown shorthand flag: 'f' in -f

After:

eksctl utils update-legacy-subnet-settings -f cluster.yaml
2022-01-10 14:06:18 [ℹ]  eksctl version 0.80.0-dev+3f6516c6.2022-01-10T14:05:34Z
2022-01-10 14:06:18 [ℹ]  using region us-west-2
....
2022-01-10 14:06:25 [ℹ]  waiting for CloudFormation changeset "eksctl-update-subnets-1641823585" for stack "eksctl-jk-cluster"
2022-01-10 14:06:44 [ℹ]  waiting for CloudFormation changeset "eksctl-update-subnets-1641823585" for stack "eksctl-jk-cluster"
2022-01-10 14:06:46 [ℹ]  waiting for CloudFormation stack "eksctl-jk-cluster"
2022-01-10 14:07:05 [ℹ]  waiting for CloudFormation stack "eksctl-jk-cluster"
2022-01-10 14:07:22 [ℹ]  waiting for CloudFormation stack "eksctl-jk-cluster"
2022-01-10 14:07:41 [ℹ]  waiting for CloudFormation stack "eksctl-jk-cluster"
2022-01-10 14:07:42 [✔]  public subnets up to date

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 🌟