Fix setup.sh to work when .nat_interface doesn't exist

[nshalman]

Description

This is a follow-up to #76 which introduced a failure:

provisioner: ./setup.sh: line 117: NAT_INTERFACE: unbound variable

Why is this needed

Unbreak setup.sh when used by Vagrant

Fixes #77

How Has This Been Tested?

I used the following simple test case. It works now that the variable is declared first, but still breaks as reported without the fix.

#!/bin/bash
set -eu
NAT_INTERFACE=""
if [ -r .nat_interface ]; then
	NAT_INTERFACE=$(cat .nat_interface)
fi
if [ -n "$NAT_INTERFACE" ] && ip addr show "$NAT_INTERFACE" &>/dev/null; then
	echo "$NAT_INTERFACE"
fi

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

Vagrant users are currently broken as reported in the community Slack.

Checklist:

I have:

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