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)
Extract our default PATH value to a constant for great reuse

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
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
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)
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
Merge pull request #4312 from shykes/engine-error-reporting

Engine: clearer job error reporting
add --line-buffered

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
Merge pull request #4290 from tianon/extract-path

Extract our default PATH value to a constant for great reuse
Merge pull request #4320 from vieux/buffer_test_integration_line_by_line

Add --line-buffered to the integration test output filtering