Add a global config option for Docker network MTU

[davidmehren]

When using this playbook in...annoying network environments, it may be required to set the MTU of Docker networks to a lower value than the default of 1500. Also annoyingly, it is not sufficient to set a default MTU using Docker daemon options, as that gets ignored when custom networks are created.

This PR introduces a new config option matrix_playbook_docker_network_mtu, that needs to get referenced every time a new Docker network is created. I currently do not know a simpler way to achieve this outcome, but I'm open to suggestions 😃

Please let me know if you'd like the option to be named differently.

[spantaleev]

I've reworked this by:

• adding a new devture_systemd_docker_base_container_networks_driver_options variable to the systemd_docker_base role (see 3cc7d1239654752da3e8f65760) - people can now use devture_systemd_docker_base_container_networks_driver_options_custom to define their own custom driver options
• updating all roles used by the playbook (both in roles/custom/) to respect the devture_systemd_docker_base_container_networks_driver_options variable
• updating various other roles pulled via Ansible Galaxy into roles/galaxy/ and making the playbook pull their new versions (via requirements.yml)

---

With this, you should be able to use this in your vars.yml configuration:

devture_systemd_docker_base_container_networks_driver_options_custom:
  com.docker.network.driver.mtu: 1234

.. and have container networks with a custom MTU of 1234 created/updated.

Adjusting devture_systemd_docker_base_container_networks_driver_options_custom subsequently is possible to a limited extent:

• changing values to existing options or introducing new options is OK
• removing options will not unset them. To unset, you either need to set to some benign value (e.g. 1500 for MTU) or to delete the networks (docker network rm $(docker network ls -q)) and have the playbook recreate them

[davidmehren]

Thanks! 🎉