Only set custom :box & :box_url values for known Bento boxes.

[fnichol]

The default values for the :box and :box_url attributes were always
implicitly based on the Bento project's base boxes
(https://github.com/chef/bento). This meant that for a small number of
common/known platforms, the box name and URLs could be pre-calculated to
help with initial out-of-the-box experience.

For example, given a platform name of "ubuntu-14.04", a default box
name of "opscode-ubuntu-14.04" and box URL could be computed (but only
for VirtualBox or VMware-based providers). Any custom platform name
would have to explicitly override the default value for at least :box
if not also for :box_url.

This commit makes the implicit default code for Bento boxes explicit. In
other words, if your platform name does not match the pre-defined Bento
box platforms, the default of :box will be the platform name and the
:box_url will be nil. Similarly, if you use a provider other than
VirtualBox or VMware-*, the default of :box will be the platform name
and the :box_url will be nil.

This update allows for less boilerplate configuration, for example:

---
driver:
  name: vagrant

provisioner:
  name: chef_zero

platforms:
  - name: ubuntu-14.04    # a Bento box, config as before
  - name: ubuntu/trusty64 # defaults to the same name for :box
  - name: windows-8.1     # defaults to the sane name for :box
                          # and a box_url of nil as there is no
                          # sharable windows base box

At a time in the near future, the S3 URLs for Bento boxes could move to
Atlas (formerly Vagrant Cloud) box names, but the same defaulting logic
can apply.