AJB-41 - gke-cluster fabric module with auto-provisioning defaults

[baliame]

Adds modified version of gke-cluster module where auto_provisioning_defaults for cluster_autoscaling are controllable.

The design choice behind making auto_provisioning_defaults a top-level input block rather than a member of the cluster_autoscaling input object variable is:

• If added, existing terraform modules which depend on this module which enable cluster_autoscaling will fail as they do not specify the new member of the object.
• This can be avoided by making the object member optional, but that is an experimental terraform feature and thus has to be explicitly enabled.
  At this point, this is the cleanest and most backwards-compatible way to update this module.

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This change is