Terraform in Equinix Metal: Fix NAT to reference correct interfaces (#76)

## Description

The NAT setup commands assume that the interface is named eth1, when clearly from the [documentation](https://github.com/tinkerbell/tinkerbell-docs/blame/master/docs/setup/equinix-metal-terraform.md#L118) it is named `enp1s0f1`. This commit fixes the NAT setup commands accordingly.

## Why is this needed

NAT doesn't work by default on Equinix Metal when following the documentation

## How Has This Been Tested?

- [x] Tested with Terraform in Equinix Metal

## How are existing users impacted? What migration steps/scripts do we need?

Existing sandboxes (that are broken) should either be rebuilt, or can run the commands manually to enable NAT

## Checklist:

I have:

- [ ] updated the documentation and/or roadmap (if required)
- [ ] added unit or e2e tests
- [ ] provided instructions on how to upgrade

deploy/terraform/main.tf

@@ -71,12 +71,9 @@ resource "null_resource" "tink_directory" {
     destination = "/root/tink"
   }
 
-  provisioner "remote-exec" {
-    inline = [
-      "iptables -A FORWARD -i eth1 -o bond0 -j ACCEPT",
-      "iptables -A FORWARD -i bond0 -o eth1 -m state --state ESTABLISHED,RELATED -j ACCEPT",
-      "iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE",
-    ]
+  provisioner "file" {
+    source      = "nat_interface"
+    destination = "/root/tink/.nat_interface"
   }
 
   provisioner "remote-exec" {

deploy/terraform/nat_interface

@@ -0,0 +1 @@
+bond0

setup.sh

@@ -110,6 +110,17 @@ setup_networking() (
 	else
 		echo "$ERR tinkerbell network interface configuration failed"
 	fi
+
+	if [ -r .nat_interface ]; then
+		NAT_INTERFACE=$(cat .nat_interface)
+	fi
+	if [ -n "$NAT_INTERFACE" ] && ip addr show "$NAT_INTERFACE" &>/dev/null; then
+		# TODO(nshalman) the terraform code would just run these commands as-is once
+		# but it would be nice to make these more persistent based on OS
+		iptables -A FORWARD -i "$TINKERBELL_NETWORK_INTERFACE" -o "$NAT_INTERFACE" -j ACCEPT
+		iptables -A FORWARD -i "$NAT_INTERFACE" -o "$TINKERBELL_NETWORK_INTERFACE" -m state --state ESTABLISHED,RELATED -j ACCEPT
+		iptables -t nat -A POSTROUTING -o "$NAT_INTERFACE" -j MASQUERADE
+	fi
 )
 
 setup_networking_manually() (