models: remove aws-k8s-1.15 files/references, redo symlinks, add aws-k8s-1.19 defaults

[etungsten]

Issue number:
N/A

Description of changes:

Author: Erikson Tung <etung@amazon.com>
Date:   Tue Apr 13 17:46:52 2021 -0700

    models: redo k8s variant symlinks, add aws-k8s-1.19 model/defaults
    
    Create default and model files for the aws-k8s-1.19 variant.
    Relink the symlink for all k8s variants so they link to aws-k8s-1.19's
    files.

Author: Erikson Tung <etung@amazon.com>
Date:   Wed Apr 14 14:09:58 2021 -0700

    variants, models: remove aws-k8s-1.15 files and references
    
    This deletes aws-k8s-1.15 models and defaults and the variant directory.
    The aws-k8s-1.15 can no longer be built.

Testing done:
Built aws-k8s-1.16, aws-k8s-1.17, aws-k8s-1.18, aws-k8s-1.19 image successfully. Launched instances with said images and they all come up successfully and show up ready in my cluster.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[jpculp]

Rather than having everything symlink to 1.16, would it make more sense to make the symlinks point to 1.19? That way we wouldn't have to make this change each time a version is deprecated.

[etungsten]

Push above removes files and directories related to aws-k8s-1.15. Truly deprecates the aws-k8s-1.15 variant.

[etungsten]

Rather than having everything symlink to 1.16, would it make more sense to make the symlinks point to 1.19? That way we wouldn't have to make this change each time a version is deprecated.

I think I'd prefer keeping the symlinks as they are now since we're likely to change something about newer k8s variants that older k8s variants might not support (necessitating a different set of defaults, setting models, etc) before we deprecate another k8s variant. So I think it makes sense to symlink back to older k8s versions as they are more "stable" in a sense.

[tjkirch]

Rather than having everything symlink to 1.16, would it make more sense to make the symlinks point to 1.19? That way we wouldn't have to make this change each time a version is deprecated.

I think I'd prefer keeping the symlinks as they are now since we're likely to change something about newer k8s variants that older k8s variants might not support (necessitating a different set of defaults, setting models, etc) before we deprecate another k8s variant. So I think it makes sense to symlink back to older k8s versions as they are more "stable" in a sense.

I don't think we'd do that with an existing variant, though, only a new one like 1.20, since we don't want to change behavior in any breaking way for existing users.

[etungsten]

Adopts #1487 (comment) suggestion.
Push above reverses the symlink's direction so aws-k8s-1.19's directory is the source of all the symlinks.

[etungsten]

Push above addresses @bcressey 's comments.