Fix NAT and make it working for Terraform and Vagrant

Commit b504810 introduced a NAT to make worker capable of reaching the
public internet via the provisioner.

But it also introduced a bug, it only works for the Vagrant setup as
Manny pointed out:

https://github.com/tinkerbell/sandbox/pull/33#issuecomment-759651035

This is an attempt to fix it

Signed-off-by: Gianluca Arbezzano <gianarb92@gmail.com>

deploy/vagrant/scripts/tinkerbell.sh

@@ -63,6 +63,12 @@ configure_vagrant_user() (
 			--password-stdin "$TINKERBELL_HOST_IP"
 )
 
+setup_nat() (
+	iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT
+	iptables -A FORWARD -i eth0 -o eth1 -m state --state ESTABLISHED,RELATED -j ACCEPT
+	iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
+)
+
 main() (
 	export DEBIAN_FRONTEND=noninteractive
 
@@ -91,6 +97,8 @@ main() (
 
 	./setup.sh
 
+    setup_nat
+
 	secure_certs
 
 	configure_vagrant_user