fix(build_image): Use developer_data as a system config.

Evaluating this as a user config causes it to block on
coreos-environment-setup.service which will wait on networking. This
makes it hard to add extra tricks for testing/debugging situations where
networking is failing. For example, to trigger dhcpcd if networkd dies:

    #cloud-config

    write_files:
      - path: /etc/systemd/system/systemd-networkd.service.d/dhcpcd.conf
        content: |
          [Unit]
          OnFailure=dhcpcd.service

          [Service]
          Restart=no