execdriver flag for docker daemon

  like the storage-driver flag, this implements a flag for choosing the
  execdriver to be used, defaulting to lxc.

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Add MTU to lxc conf to make host and container MTU match

If you are using jumbo frames the host side of the veth was being set to
1500.  Docker would set the MTU of the container side of the veth to 9001.
This would lead to a situation in which the two sides of the veth had
different MTU sizes causing issues in network traffic.

Docker-DCO-1.1-Signed-off-by: Darren Shepherd <darren.s.shepherd@gmail.com> (github: ibuildthecloud)
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 trailing slash from host parameter in docker init.

(This causes errors on Docker 0.8)

Docker-DCO-1.1-Signed-off-by: Morgante Pell <morgante.pell@morgante.net> (github: morgante)
Remove uneeded wording in Dockerfile format

Docker-DCO-1.1-Signed-off-by: Brian Goff <cpuguy83@gmail.com> (github: cpuguy83)
seperate out the terminal functions from lxc to the pkg/term

  Since these functions are indepenent of lxc, and could be used by
  other drivers.

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
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)
api/common.go: code shared by the server and client implementation of the Docker remote api.

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
Move the terminal setup to execdriver, instead of ./pkg/term

  It is independent of any particular driver, but likely used by
  multiple execdrivers. Also, pkg/... is not to have any links to
  docker, which this terminal setup does.

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Engine: cleanup side effects between tests

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