Validate backend host count on config init

[thedae]

Fixes krakend/krakend-ce#948

Config initialization fails if there's an endpoint without any backend hosts

Example

Config file

{
  "$schema": "https://www.krakend.io/schema/krakend.json",
  "version": 3,
  "echo_endpoint": true,
  "endpoints": [
    {
      "endpoint": "/some-api",
      "backend": [
        {
          "url_pattern": "/__echo",
          "host": [
            "http://localhost:8080"
          ]
        }
      ]
    },
    {
      "endpoint": "/api-without-backend",
      "backend": [
        {
          "url_pattern": "/__echo",
          "host": []
        }
      ]
    }
  ]
}

Config check

$ krakend check -c ./krakend.json 
Parsing configuration file: /home/dae/work/krakend/configs/ce-issue-948/krakend.json
ERROR parsing the configuration file:	'/home/dae/work/krakend/configs/ce-issue-948/krakend.json': can't register 'GET /api-without-backend' endpoint, it doesn't have any host

exit status 1

Startup

$ krakend run -c ./krakend.json 
ERROR parsing the configuration file: '/home/dae/work/krakend/configs/ce-issue-948/krakend.json': can't register 'GET /api-without-backend' endpoint, it doesn't have any host
exit status 255

[taik0]

This has been addressed in the schema and the check command of KrakenD. We will maintain the current behavior.