Supporting multi-dc deploys

[mdkent]

This is an early sketch for discussion.

This branch:

• Enables aliases+anchors in yaml.
  • This is huge for DRY within the layered config.yaml and config.staging.yaml to make controlling the large number of roles to sustain a multi-site deployment realistic.
• Drops the requirement to provide a :web role. Let's just use whatever role is running traefik and call it good enough.
  • This lets us name all of the roles after a datacenter - rather than carrying one awkward :web role.
• Drops the requirement for roles to have hosts. The fact they're empty is harmless in the end if the user is informed.
  • This let us toggle roles on/off for partial deployments. Very useful in outages.

[mdkent]

I've broken this out into

#577
#578
#582
#583