Update readme with more details on Amazon Linux repo_upgrade race issue

[sparrowt]

This adds more explanatory text, and also a specific example of how to set repo_upgrade: none via #cloud-config in user data.

I spent a long time trying to modify the cloud-init config within the AMI before realising I could just set the 'User data' on the Jenkins Clouds config for the AMI, so hopefully this will help someone else too.

Also list other symptoms of the same problem when Docker is being used which results in EOF error messages - it took me quite a while to realise that this was due to Amazon Linux security patching so again hopefully this reference will help others facing the same issue.

Testing done

This is a readme only change - however I have tested that this snippet of 'User Data' in the Jenkins Clouds AMI configuration successfully results in cloud-init not running the yum upgrade command that was previously being run. I now see this step being essentially a no-op in /var/log/cloud-init.log

Nov 08 16:03:49 cloud-init[3626]: handlers.py[DEBUG]: start: modules-config/config-package-update-upgrade-install: running config-package-update-upgrade-install with frequency once-per-instance
Nov 08 16:03:49 cloud-init[3626]: util.py[DEBUG]: Writing to /var/lib/cloud/instances/i-INSTANCEID/sem/config_package_update_upgrade_install - wb: [644] 20 bytes
Nov 08 16:03:49 cloud-init[3626]: helpers.py[DEBUG]: Running config-package-update-upgrade-install using lock (<FileLock using file '/var/lib/cloud/instances/i-INSTANCEID/sem/config_package_update_upgrade_install'>)
Nov 08 16:03:49 cloud-init[3626]: handlers.py[DEBUG]: finish: modules-config/config-package-update-upgrade-install: SUCCESS: config-package-update-upgrade-install ran successfully

whereas previously it actually did something (run yum) between "Running" and "finish" e.g. like this:

Nov 08 15:02:55 cloud-init[3600]: handlers.py[DEBUG]: start: modules-config/config-package-update-upgrade-install: running config-package-update-upgrade-install with frequency once-per-instance
Nov 08 15:02:55 cloud-init[3600]: util.py[DEBUG]: Writing to /var/lib/cloud/instances/i-INSTANCEID/sem/config_package_update_upgrade_install - wb: [644] 20 bytes
Nov 08 15:02:55 cloud-init[3600]: helpers.py[DEBUG]: Running config-package-update-upgrade-install using lock (<FileLock using file '/var/lib/cloud/instances/i-INSTANCEID/sem/config_package_update_upgrade_install'>)
Nov 08 15:02:55 cloud-init[3600]: amazon.py[DEBUG]: Upgrade level: security
Nov 08 15:02:55 cloud-init[3600]: rhel.py[DEBUG]: Using YUM for package management
Nov 08 15:02:55 cloud-init[3600]: util.py[DEBUG]: Running command ['yum', '-t', '-y', '--exclude=kernel', '--exclude=nvidia*', '--exclude=cuda*', '--security', '--sec-severity=critical', '--sec-severity=important', 'upgrade'] with allowed return codes [0] (shell=False, capture=False)
Nov 08 15:03:18 cloud-init[3600]: handlers.py[DEBUG]: finish: modules-config/config-package-update-upgrade-install: SUCCESS: config-package-update-upgrade-install ran successfully

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