Remove Vagrantfile and remove it from all docs

This removes the Vagrantfile and updates the documentation to remove
the steps which explain how to install Docker in a VM via Vagrant.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #4281 from unclejack/remove_vagrantfile

Remove Vagrantfile and remove it from all docs
Engine: clearer job error reporting

* Job.Error and Job.Errorf always print a newline.
* Avoid redundant prefixing of error messages.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)