Develop and adopt boot-time fix for EOL CentOS 7 repositories

[tpdownes]

This PR introduces a new runner intended to apply early-boot hotfixes on VMs that use our startup-script module. It is chained as 3 commits:

1. The early boot hotfix itself
2. Adoption of the hotfix in the vm-instance module's use of startup-script
3. Un an integration test that will make use of 1+2, swap the centos-7 VM family name for image ID because all images in the family are now deprecated (i.e. you can't use the family name to get latest non-deprecated image)

Observed in logs:

Jul  2 22:31:09 testfix-0 startup-script-stdlib[1236]: === start executing runner: early_run_hotfixes.sh-b383 ===
Jul  2 22:31:09 testfix-0 google_metadata_script_runner: startup-script: Tue Jul 02 22:31:09 +0000 2024 Info [1236]: === start executing runner: early
_run_hotfixes.sh-b383 ===
Jul  2 22:31:09 testfix-0 google_metadata_script_runner: startup-script: Applying hotfixes for CentOS 7
Jul  2 22:31:09 testfix-0 google_metadata_script_runner: startup-script: Removing mirrorlist from default CentOS 7 repositories
Jul  2 22:31:09 testfix-0 google_metadata_script_runner: startup-script: Reconfiguring default CentOS 7 repositories to use CentOS Vault
Jul  2 22:31:09 testfix-0 startup-script-stdlib[1236]: === early_run_hotfixes.sh-b383 finished with exit_code=0 ===

On a repeated run, you (correctly) only observe the first message:

Jul  2 22:32:51 testfix-0 startup-script-stdlib[1811]: === start executing runner: early_run_hotfixes.sh-b383 ===
Jul  2 22:32:51 testfix-0 google_metadata_script_runner: startup-script: Tue Jul 02 22:32:51 +0000 2024 Info [1811]: === start executing runner: early_run_hotfixes.sh-b383 ===
Jul  2 22:32:51 testfix-0 google_metadata_script_runner: startup-script: Applying hotfixes for CentOS 7

Submission Checklist

Please take the following actions before submitting this pull request.

• Fork your PR branch from the Toolkit "develop" branch (not main)
• Test all changes with pre-commit in a local branch #
• Confirm that "make tests" passes all tests
• Add or modify unit tests to cover code changes
• Ensure that unit test coverage remains above 80%
• Update all applicable documentation
• Follow Cloud HPC Toolkit Contribution guidelines #