Use Bento's Atlas boxes by default if detected

[andytson]

Bento hosts its base boxes on https://atlas.hashicorp.com/bento

I'm unsure whether it's worth removing default_box_url, but from a class-api BC, keeping this in will still allow for monkey-patching new behaviours in.

Fixes #193

[legal90]

It is a good suggestion and it allows to do not care about box urls at all. It also supersedes my previous PR #196
But please note, that actual box name format <atlas_username>/<box_name> is available only for Vagrant 1.5+ (http://www.vagrantup.com/blog/vagrant-1-5-and-vagrant-cloud.html)

Currently README tells that the minimal supported version of Vagrant is 1.1, so it should be changed to 1.5 after this pull request.

[legal90]

@tyler-ball @fnichol Please, take a look at this PR.

[davidski]

Would love to see this merged and released. Current test-kitchen behavior causes opscode-* boxes to be downloaded and preferred over the atlas-hosted bento boxes, duplicating storage and loosing out on version updates.

[cheeseplus]

all the 👍

[tas50]

Looking forward to this one. Thanks everyone