Support docker.elastic.co registry

[bgagnon]

This popular registry hosts all official images for Elastic products: https://docker.elastic.co

It looks like it supports a standard V2 API, for example: https://docker.elastic.co/v2/elasticsearch/elasticsearch/tags/list

[arminc]

@bgagnon I have couple of registries inside the code now but when I am finished with refactoring I would like to put them all in the config instead of in the code. Having said that I will add this one to that config list as well. If you need to use this registry now, just know that you can, you can configure any registry in the config your self already.