Update default route interface lookup: (#70)

## Description


If multiple default routes exists, we should only use the first one.

## Why is this needed



Fixes: #

## How Has This Been Tested?





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





## Checklist:

I have:

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

tinkerbell/stack/templates/nginx.yaml

@@ -116,7 +116,7 @@ spec:
             # if sourceInterface is not set use the interface from the default route
             srcInterface="{{ $sourceInterface }}"
             if [ -z "$srcInterface" ]; then
-              srcInterface=$(nsenter -t1 -n ip route | awk '/default/ {print $5}')
+              srcInterface=$(nsenter -t1 -n ip route | awk '/default/ {print $5}' | head -n1)
             fi
             # Create a macvlan interface. TODO: If this fails, try again with a different name?
             nsenter -t1 -n ip link add {{ $macvlanInterfaceName }} link ${srcInterface} type macvlan mode bridge