Add auto_delete and instance_redistribution_type to compute-vm and compute-mig modules.

[giovannibaratta]

While working with compute-vm and compute-mig modules, I encounted the following issues:

• the default value for auto_delete used by google_compute_instance_template creates a conflict with READ_ONLY disks (ex. https://github.com/GoogleCloudPlatform/cloud-foundation-fabric/tree/master/modules/compute-mig#stateful-migs---mig-config ) and it's not possible to customize the attribute in the module compute-vm, therefore it's not possible to create disk with read only disks with the current version of the module.
• the default value for instance_redistribution_type used by google_compute_region_instance_group_manager is not compatible with stateful migs and it's not possible to customize the attribute in the module compute-mig, therefore it's not possible to create regional stateful mig with the current version of the module (if I did not miss anything).
• (minor) I tried the code in the example Stateful MIGs - MIG Config in modules/compute-mig/README.md the block stateful_config and it's not working in my case. I guess there is an error.

This PR add the two attributes in the respective module and let the user to customize them in order to implement the mentioned use case.

---

Using a template with attached_disk of type read_only in compute-vm results in:

│ Error: Error creating instance template: googleapi: Error 400: Invalid value for field 'resource.properties.disks[1].autoDelete': 'true'. autoDelete can only be specified on read-write disks., invalid
│
│   with module.nginx-template.google_compute_instance_template.default[0],
│   on ../repos/cloud-foundation-fabric/modules/compute-vm/main.tf line 254, in resource "google_compute_instance_template" "default":
│  254: resource "google_compute_instance_template" "default" {

Using a regional stateful mig in compute-mig results in:

│ Error: Error creating RegionInstanceGroupManager: googleapi: Error 400: Invalid value for field 'resource.updatePolicy.instanceRedistributionType': 'UNSPECIFIED_TYPE'. Stateful features can be used only when instance redistribution is disabled (set to NONE)., invalid
│
│   with module.nginx-mig.google_compute_region_instance_group_manager.default[0],
│   on ../repos/cloud-foundation-fabric/modules/compute-mig/main.tf line 235, in resource "google_compute_region_instance_group_manager" "default":
│  235: resource "google_compute_region_instance_group_manager" "default" {

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[ludoo]

Giovanni, thanks for the detailed explanation and code changes. Can you run the tfdoc tool to fix documentation linting errors? Our contributing guide has the details.

[ludoo]

Thanks a lot for this!