Added caddy as swarm container. Uses templated Caddyfile which can be
set up for testing purposes for now.
Depends on docker and docker-swarm, and will keep its stuff where
docker-swarm sets itself up.
Variable `docker_stack_files_dir` can be used if other stack deployments
need to copy over configuration files or other extra files. Defaults to
`/stacks` but can be set to anything and other roles should take care to
respect this and put their files relative to it.
Variable `docker_swarm_public_network_name` can be used to set the name
of the network to be created that faces outwards (ingress network) and
to which all stacks will be connecting.
Added variable docker_swarm_advertise_addr which can be set to the
advertise address swarm should use.
Mostly introduced for vagrant-based test setups for now, which contain
multiple IP addresses and fail otherwise.
