Add swarm prod example

[rachedbenmustapha]

This docker service stack describes a simple Zenko production setup, including:

• Host filesystem-based, easy to back up volumes for data and metadata
• Load balancer (nginx-based) on all nodes of the swarm
• Multi-tiered networks (user-facing, DMZ and backend services)
• Thanks to Docker Swarm and its overlay network, virtual ips and scheduler,
  high availability and service resiliency (storage node excluded).

Needs scality/S3#777

[LaurenSpiegel]

can we use swarm secrets for these?

[rachedbenmustapha]

I forgot to mention it, thanks. I added it to the "further work" section because it will need another update to S3's docker-entrypoint.sh

[LaurenSpiegel]

mention redis too?

[rachedbenmustapha]

Since what we store in redis is thrown away every 30s (or is it?), I think fits into the "resiliency" category since swarm scheduler will do its job of keeping one alive :)