Support customizing the primary_web_role

[mdkent]

This adds support for controlling the primary_web_role instead of always defaulting to web.

This allow for more flexible role naming in configs, eg: web_chicago.

Example config:

servers:
  web_chicago:
    traefik: true
    hosts:
      - 192.168.0.1
      - 192.168.0.2
  web_tokyo:
    traefik: true
    hosts:
      - 192.168.0.3
      - 192.168.0.4

[mdkent]

Could rework #580 after merging this.