Use box_url unless box is a cloud shorthand name
Added box_url spec to init_generator_spec.rb
Removed attribute vagrant.vm.box_url from default config
Don't add vm.box_url to vagrant unless set by user
Remove box_url from unrelated specs
Set box_url default to nil and print deprecation message
Merge pull request #1332 from jossy/master

Vagrant: Use vm.box_url when vm.box is not a Vagrant Cloud box