AWSMachinePool controller should clean up unused Launch Template Versions

[dlipovetsky]

/kind bug

Describe the solution you'd like
The AWSMachinePool controller creates new Launch Template Versions when the configuration changes, or the MachinePool is scaled. (See #2354). AWS imposes a limit on the number of Launch Template Versions:

You are limited to creating 5,000 launch templates per Region and 10,000 versions per launch template.
-- https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-launch-templates.html#launch-template-restrictions

When this limit is reached, further updates are not possible, and therefore further configuration changes or scale operations would be blocked.

[dlipovetsky]

I'm worried that I did not raise this in the v0.6.6 release notes. I think that there's a new version created every time a new bootstrap token is created; assuming that's about every 10m, then a pool can operate for about 10m * 10,000 versions = 100,000m, or approximately 69 days, before its launch template reaches the maximum allowed versions.

[sedefsavas]

Would you like to address the cleanup in the v0.6.7 release?

I will add this as a warning to the v0.6.6 for new users. (update: added)

[dlipovetsky]

I will add this as a warning to the v0.6.6 for new users. (update: added)

Thank you, @sedefsavas!

Would you like to address the cleanup in the v0.6.7 release?

I will try.

[k8s-ci-robot]

@dlipovetsky: You must be a member of the kubernetes-sigs/cluster-api-provider-aws-maintainers GitHub team to set the milestone. If you believe you should be able to issue the /milestone command, please contact your Cluster API Provider AWS Maintainers and have them propose you as an additional delegate for this responsibility.

In response to this:

/milestone v0.7

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