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)
Remove uneeded wording in Dockerfile format

Docker-DCO-1.1-Signed-off-by: Brian Goff <cpuguy83@gmail.com> (github: cpuguy83)
Merge pull request #4293 from cpuguy83/patch-1

Doc: Remove uneeded wording in Dockerfile format
Register built-in engine commands at runtime instead of compile-time

This allows selective loading of commands, and paves the way to dynamic
plugins.

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

Refactor integration tests to remove special cases in the creation of
test engines. All test engines are now created through newTestEngine.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Engine.ParseJob: create a new job from a shell-like text command.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Merge pull request #4281 from unclejack/remove_vagrantfile

Remove Vagrantfile and remove it from all docs
Merge pull request #4249 from shykes/builtins

Register built-in engine commands at runtime instead of compile-time
Add "SteamOS" support to mkimage-debootstrap (requires a debootstrap script like https://github.com/tianon/gentoo-overlay/blob/master/dev-util/debootstrap-valve/files/scripts/alchemist)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4311 from shykes/engine-parsejob

Engine.ParseJob: create a new job from a shell-like text command.
Merge pull request #4288 from tianon/mkimage-steamos

Add "SteamOS" support to mkimage-debootstrap