staging: Modularize node creation

This allows to refactor the redundancy between nodes.

I kept out of the refactor loop the gateway for now because it's diverging a bit much with other nodes (I'm not fluent enough yet with terraform dsl to understand how i can make it comply)

In the next commit (D1807), we'll see how to use that refactoring to declare a new node without too many duplication.

@douardda might be interested by it since we talked about it ;)

Test Plan

terraform init
terraform plan

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Migrated from D1806 (view on Phabricator)