Berkshelf gem should not depend on Vagrant gem

If Vagrant is installed via the standalone Vagrant installer, it contains its own embedded ruby instance, which may be a different Ruby context than the one in which Berkshelf has been installed.

This can lead to multiple, conflicting copies and versions of Vagrant installed on the same host, resulting in end-user confusion.

Let the user decide if Vagrant should be installed via the same Ruby context as Berkshelf, or if the standalone 'full-stack' Vagrant installer should be used instead.