Install image-gc configurable will sane defaults

[nalthoff]

What this PR does / why we need it:

This enhances the k8s cluster definition to allow the user to set custom values for --image-gc-high-threshold and --image-gc-low-threshold. If the user doesn't set the values, the default k8s values are explicity set (85/80)

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #1374

Special notes for your reviewer:

Documentation updated as well. Unit tests were not written, but if needed please let me know where/how

Release note:

NONE

[msftclas]

@nalthoff,
Thanks for having already signed the Contribution License Agreement. Your agreement was validated by Microsoft. We will now review your pull request.
Thanks,
Microsoft Pull Request Bot

[SaaldjorMike]

According to https://kubernetes.io/docs/admin/kubelet/ the default high threshold is:

--image-gc-high-threshold int32 The percent of disk usage after which image garbage collection is always run. (default 85)

but according to the docs at https://kubernetes.io/docs/concepts/cluster-administration/kubelet-garbage-collection/ it seems that it is 90%?

image-gc-high-threshold, the percent of disk usage which triggers image garbage collection. Default is 90%.

Did I miss something or is one of those pages out of date?

EDIT: Apparently it was previously lowered from 90% to 85%, see kubernetes/kubernetes#40432

[nalthoff]

@SaaldjorMike Yup. I dug into the defaults for 1.5, 1.6 and 1.7 and decided to go with the 1.7 defaults for all of them since they were pretty similar.

[nalthoff]

@lachie83 Here is the PR for the issue you created for me.

[jackfrancis]

Thanks @nalthoff and @SaaldjorMike !

[lachie83]

Thanks for taking care of this @nalthoff. Very much appreciated!