Move cleanup to separate provisioner, remove cleanup_image var

[cartermckinnon]

Description of changes:

Performing the cleanup steps within install-worker.sh is problematic, because they can only be skipped -- they can't be invoked separately from the rest of install-worker.sh. This is a pain for our GPU AMI template, for example.

The cleanup_image var was added to the packer template in #522. The value is passed to install-worker.sh as an environment variable, CLEANUP_IMAGE, which toggles the removal of various bits from the resulting AMI.

However, this variable doesn't actually work with our packer template; if you set its value to false, the build will fail. This is because validate.sh requires the cleanup steps to be run. This toggle would only make sense if you were using install-worker.sh in a custom packer template (which I assume is what folks are doing). However, I don't think it makes sense to allow cleanup to be disabled in the packer template that we publish, because validate.sh should never be optional in this template.

Removing the variable from our template won't break users who are using install-worker.sh in a custom template and setting CLEANUP_IMAGE=false. These users wouldn't include the new cleanup.sh provisioner; so there is no functional change.

This could break users who are directly modifying our packer template at build-time, such as setting cleanup_image to false and then deleting the validate.sh provisioner. This type of usage is inherently fragile and we can't always avoid breaking it. It shouldn't be difficult to resolve.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

Testing Done

Before this change, this will fail:

# cleanup_image was never added to PACKER_VARIABLES in the Makefile, so just setting it here
> echo $(jq '.variables.cleanup_image = "false"' eks-worker-al2.json) > eks-worker-al2.json
> make 1.23
...
2022-09-28T14:57:52-07:00: ==> amazon-ebs: Provisioning with shell script: /var/folders/5t/c97ylshx62gb8ctzrmbtbv4h0000gs/T/tmp.AUBkWKrX/amazon-eks-ami/scripts/validate.sh
2022-09-28T14:57:52-07:00:     amazon-ebs: /etc/hostname shouldn't exists
...

After this change, this succeeds:

> make 1.23

[cartermckinnon]

@ericlarssen-wf if you can add details about how you've used cleanup_image, it'd be helpful.

[mmerkes]

Is this an important thing?

[mmerkes]

NVM, looks like you address in your description.

[mmerkes]

NVM, looks like you address in your description.