Allow routes to override variables taken from `config.json`

[ajxchapman]

Allow individual routes to override the variables defined in config.json, this could help with things like allowing different webhooks endpoints based on the route, rather than the generic definition from config.json

[ajxchapman]

This can already be achieved by passing arguments to scripts using the kwargs definition, e.g.

"action" : {
  "module" : "./scripts/slack_alert_middleware/slack_alert.py",
  "function" : "http_alert",
  "kwargs" : {
    "webhook" : "{{slack_webhook_url}}"
  }
}

Is there any benefit of implementing an additional override?