Rename routes_path to router_config
Rename routes_path to router_config

I think `routes_path` is from a time when we used to specify our
routes in the Nginx config rather than use the GOV.UK Router.

`router_config` seems a more apt name.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet