remove count in favor of sets

[hareware-labs]

• In this module, as it stands, if additional vpc subnets are added, google_compute_subnetwork_iam_member resources are deleted and recreated.
• Switching to lists preserves resources during applies where additional subnets are added.
• This change removes count in favor of the for_each / toset construct so that order is maintained during applies.

[google-cla]

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