Add support for external docker networks

[jessesimpson36]

When having other docker compose files that submit jobs to the lava server within this docker compose file, it's useful to be able to set the external docker network.

This change introduces a new option that allows you to specify external_docker_networks in a list and for each network name you add to your config file, your services will be connected to that docker network.

For my example, I had a network named gkci-main_gkernelci-net, so I configured boards.yaml with

masters:
  - name: master1
...
    docker_external_networks:
      - "gkci-main_gkernelci-net"
...
slaves:
  - name: lab-slave-0
    docker_external_networks:
      - "gkci-main_gkernelci-net"
...

And the resulting compose file has :

networks:
  gkci-main_gkernelci-net:
    external: true

services:
  lab-slave-0:
...
    networks:
    - gkci-main_gkernelci-net
 ...
  master1:
    networks:
    - gkci-main_gkernelci-net
...