Create NetBox sites rather than requiring them to pre-exist

The syd1 site does not exist, so the site data source failed the plan.
Sites now follow the same declared-by-reference rule as device roles,
types and manufacturers.

README.md

@@ -27,6 +27,10 @@ ci/puppetdb_backfill.py                     # reality backfill generator
 `make plan` / `make apply` reconcile the YAML against NetBox. State lives in
 Consul under `infra/terraform/netbox/<region>/<dc>/state`.
 
+Sites, device roles, device types and manufacturers are all declared by being
+referenced: any slug a subnet or device names is created. A site that already
+exists in NetBox must be imported, not re-created, since site names are unique.
+
 ## Subnet schema (`subnets/<name>.yaml`)
 
 ```yaml

modules/netbox/devices.tf

@@ -52,7 +52,7 @@ resource "netbox_device" "this" {
   name           = each.key
   device_type_id = netbox_device_type.this[each.value.model_hint].id
   role_id        = netbox_device_role.this[each.value.role].id
-  site_id        = tonumber(data.netbox_site.this[each.value.site].id)
+  site_id        = tonumber(netbox_site.this[each.value.site].id)
   status         = each.value.pxe ? "staged" : "active"
 
   platform_id = try(netbox_platform.this[each.value.provision.platform].id, null)

modules/netbox/main.tf

@@ -28,9 +28,15 @@ locals {
   }
 }
 
-data "netbox_site" "this" {
+# Sites are declared by being referenced: every slug named by a subnet or a
+# device is created, exactly as device roles, types and manufacturers are. A
+# site that already exists in NetBox must be imported rather than re-created
+# (NetBox site names are unique).
+resource "netbox_site" "this" {
   for_each = local.sites
-  slug     = each.value
+
+  name = each.value
+  slug = each.value
 }
 
 # One tag per network so prefixes can be discovered by network membership.
@@ -47,7 +53,7 @@ resource "netbox_prefix" "this" {
   prefix      = each.value.prefix
   status      = "active"
   description = each.value.description
-  site_id     = each.value.site != null ? tonumber(data.netbox_site.this[each.value.site].id) : null
+  site_id     = each.value.site != null ? tonumber(netbox_site.this[each.value.site].id) : null
   tags        = [for t in local.prefix_net_tags[each.key] : t]
 
   depends_on = [netbox_tag.network]