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

---

Migrated from D1806 (view on Phabricator)

proxmox/terraform/modules/node/main.tf

+resource "proxmox_vm_qemu" "node" {
+    name              = "${var.hostname}"
+    desc              = "${var.description}"
+    # hypervisor onto which make the vm
+    target_node       = "${var.hypervisor}"
+    # See init-template.md to see the template vm bootstrap
+    clone             = "${var.template}"
+    # linux kernel 2.6
+    qemu_os           = "l26"
+    # generic setup
+    sockets           = "${var.sockets}"
+    cores             = "${var.cores}"
+    memory            = "${var.memory}"
+    # boot machine when hypervirsor starts
+    onboot            = true
+    # cloud-init setup
+    os_type           = "cloud-init"
+    # ciuser - User name to change ssh keys and password for instead of the
+    # image’s configured default user.
+    ciuser            = "${var.user_admin}"
+    ssh_user          = "${var.user_admin}"
+    # sshkeys - public ssh keys, one per line
+    sshkeys           = "${var.user_admin_ssh_public_key}"
+    # searchdomain - Sets DNS search domains for a container.
+    searchdomain      = "${var.domain}"
+    # nameserver - Sets DNS server IP address for a container.
+    nameserver        = "${var.dns}"
+    # ipconfig0 - [gw =] [,ip=<IPv4Format/CIDR>]
+    ipconfig0         = "ip=${var.network["ip"]}/24,gw=${var.gateway_ip}"
+    disk {
+        id           = 0
+        type         = "virtio"
+        storage      = "${var.storage["location"]}"
+        storage_type = "ssd"
+        size         = "${var.storage["size"]}"
+    }
+    network {
+        id      = 0
+        model   = "virtio"
+        bridge  = "vmbr0"
+        macaddr = "${lookup(var.network, "macaddr", "")}"
+    }
+
+    # Delegate to puppet at the end of the provisioning the software setup
+    provisioner "remote-exec" {
+        inline = [
+            "sed -i 's/127.0.1.1/${var.network["ip"]}/g' /etc/hosts",
+            "puppet agent --server ${var.puppet_master} --environment=${var.puppet_environment} --waitforcert 60 --test || echo 'Node provisionned!'",
+        ]
+    }
+}