Add docker.elastic.co support for elasticsearch images

[danielebarbaro]

Hi!
This PR solve #76 .

Official elasticsearch images are in https://www.docker.elastic.co/r/elasticsearch.
The problem was that https://docker.elastic.co/v2/elasticsearch/elasticsearch/tags/list needs an access_token.

I wrote some lines of code to catch a valid access_token before call tag list url.

[abenerd]

This looks great!
Do you know how long the access token is valid for? Maybe if it is for long enough we could cache it for a time so we don't have to request it always.

[danielebarbaro]

Hi @abenettt, unfortunately there's no expire_in field on auth response.

I've been thinking about saving the token somewhere but you need to take a valid token only when you create a new container.

[abenerd]

Ah yeah you're right it would not really make sense to save the token.

[danielebarbaro]

hi, @josecanhelp @mattstauffer I just updated the PR just to keep branch cleaned. Feel free to discard this PR

[josecanhelp]

Great work, @danielebarbaro. Thanks!