Add experimenta btrfs driver

This is an experimental btrfs driver. To use it you must have
/var/lib/docker mounted on a btrfs filesystem and explicitly
specify DOCKER_DRIVER=btrfs in the docker daemon environment.

It works by using subvolumes for the docker image/container layers.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
btrfs: Add comment to Put()

Document why we don't need to do anything in Put().

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
add tests

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
fix convertion issues

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
 Add troubleshooting for missing cgroups on mint 16 #3602

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
network: insert masq rule

This fixes IP masquerading on systems with reject rules at the end of
the POSTROUTING table, by inserting the rule at the beginning of the
table instead of adding it at the end.

Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)
Merge pull request #3718 from alexlarsson/btrfs

Add btrfs support to docker
Make unclejack the official Vagrantfile maintainer

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3846 from tianon/unclejack-maintainer

Make unclejack the official Vagrantfile maintainer
Merge pull request #3836 from SvenDowideit/mint-cgroups-lite-docs

 Add troubleshooting for missing cgroups on mint 16
Merge pull request #3842 from jpoimboe/iptables-insert-masq

network: insert masq rule
refactor all *FromJob functions

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
Merge pull request #3833 from vieux/fix_env_conversion_issues

Fix env conversion issues in env