feat: Add option image_type in cluster_autoscaling block

[endrec]

Fixes #1904

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[apeabody]

/gcbrun

[apeabody]

Thanks @endrec! - Can you please (also) make your change to the template: autogen/main/variables.tf.tmpl and then make build.

[endrec]

@apeabody Done. :)

[apeabody]

/gcbrun