Capture outbound routing config in emis-access backend

[bloodearnest]

emis-access just manages users, no services.

However, it does need to ensure that its network routing is configured to use its assigned static IP for all outbound traffic.

This looks like adding the following to /etc/netplan/50-cloud-init.yaml config for eth0:

          routes:
              # as per https://docs.digitalocean.com/products/networking/reserved-ips/how-to/outbound-traffic/
              - to: 0.0.0.0/0
                via: 10.16.0.1

It may be that emis network access is changing, and we won't need this