Under certain conditions when watchtower is monitoring a Docker Swarm
cluster there would be cases where an updated container could not be
started because the old hadn't yet been removed (name conflicts, mapped
port conflicts, etc). We suspect that this has something to do with the
async nature of swarm and even though we've asked the swarm master to
remove a container it may not be completely removed from the associated
node. The fix is to do some polling after the remove container call to
ensure that the container is truly gone before proceeding.
