Add support for specifying docker swarm subnet ranges about ansible-role-docker

[tomdaley92]

When I "refresh" my VMs next, this will be a good time to do this, due to the fact that you can only change the subnet ranges during a docker swarm init.

[tomdaley92]

per docs this is acheived by creating the docker network before running docker swarm init

For example the order would be:

sudo apt install docker-ce

docker network create \
--subnet=10.200.0.0/16 \
--gateway 10.200.0.1 \
-o com.docker.network.bridge.enable_icc=false \
-o com.docker.network.bridge.name=docker_gwbridge \
-o com.docker.network.bridge.enable_ip_masquerade=true \
docker_gwbridge

docker swarm init