Add a few small tweaks to mkimage-debootstrap, especially to help with lucid

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
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)
Added new mkimage-yum.sh script to create CentOS base images

mkimage-rinse.sh requires rinse, which is not readily available on
CentOS or Fedora.  Plus, creating a base image is trivial with yum
alone.

Docker-DCO-1.1-Signed-off-by: Chris St. Pierre <chris.a.st.pierre@gmail.com> (github: stpierre)
fix shebang in mkimage-yum.sh

Docker-DCO-1.1-Signed-off-by: Chris St. Pierre <chris.a.st.pierre@gmail.com> (github: stpierre)
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)
execdriver: Make GetPidsForContainer() a driver call

The current implementation is lxc specific.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
network: add publicly mapped ports to FORWARD table

Allow publicly mapped ports to be made public beyond the host.  This is
needed for distros like Fedora and RHEL which have a reject all rule at
the end of their FORWARD table.

Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)
docs: remove ip_forward enabling step

Now that docker sets /proc/sys/net/ipv4/ip_forward by default, remove
the step to enable it in the installation docs.

Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)
Initial move of port mapper code into sub pkg

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
remove enableCors from the config and move it as a job arg

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
Move port mapper tests out of core and into portmapper

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
move warnings to the job

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
Merge pull request #3813 from jpoimboe/update-ip_forward-docs

docs: remove ip_forward enabling step
ExecStartPre commands updated

Docker-DCO-1.1-Signed-off-by: Lokesh Mandvekar <lsm5@redhat.com> (github: lsm5)

systemd service no longer does '/bin/mount/ --make-rprivate /'.
Core issue fixed by Alex Larsson (commit 157d99a).

ip forwarding enabled.
remove ip fowarding from systemd (fixed: commit #3801)

Docker-DCO-1.1-Signed-off-by: Lokesh Mandvekar <lsm5@redhat.com> (github: lsm5)
Merge pull request #3588 from tianon/debootstrap-tweaks

Add a few small tweaks to mkimage-debootstrap, especially to help with lucid
Merge pull request #3820 from vieux/remove_runtime_from_api

Remove runtime from api
Merge pull request #3808 from alexlarsson/execdriver-get-pids-for-container

execdriver: Make GetPidsForContainer() a driver call
Merge pull request #3819 from crosbymichael/move-port-mapper

Move port mapper out of core and into network drivers
Merge pull request #3810 from jpoimboe/iptables-forward-public-ports

network: add publicly mapped ports to FORWARD table