xenial vagrant boxes in Atlas all resolve to same version

When adding the xenial64 vagrant box via Atlas, you will always get the latest box version, even if you specify an older version.

So,

vagrant box add ubuntu/xenial64

and

vagrant box add ubuntu/xenial64 --box-version SOMEOTHERVERSION

will both install the latest version.

This may surprise some users or cause issues for build pipelines that would expect to be able to retrieve older versions using Atlas.

As per <https://bugs.launchpad.net/cloud-images/+bug/1590647/comments/1>, the reason this currently happens is because the Atlas boxes are redirected to the current box version.