Add support for additional block device for docker

[alanbchristie]

Expose a variable that defines the size of an additional block device to be used for docker storage for each node. This step can be done after installing Docker and should be done before creating images or containers.

• terraform would create the devices (per node)
• a pre-openshift playbook would disable docker, setup the device and re-enable docker

See Configuring Thin Pool Storage in https://docs.okd.io/3.9/install_config/install/host_preparation.html#configuring-docker-storage