By default, swarm services use ingress mode port publishing, which is
not ideal for traefik (it breaks IPv6 ingress and there is no need to
load-balance traffic between multiple traefik instances or to route it
from multiple swarm nodes)
This commit switches traefik's port publishing mode to `host` for all of
its exposed ports as well as:
* change traefik's update order to stop-first (there cannot be multiple
containers exposing the same port when using host-mode publishing)
* use `endpoint_mode: dnsrr` instead of the default `vip`
* remove all overrides from `compose.host.yml`, leaving the file empty
for backwards compatibility
* allow overriding traefik's endpoint_mode back to vip through
compose.no-host.yml
/!\ This is a breaking change
Closes: #52
