Add synchronization between jobs and server closing

Fixes #5154
Daemon waiting 15 seconds for finishing server jobs before shutdown. In
this time it doesn't accept new jobs. After this time, it shutdown
despite running jobs.
Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)
Add wait flag to iptables

Fixes #1573
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
We need to umount /var/lib/docker when the daemon exits.

Currently we are leaving it bind mounted on stop.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
Update integration tests with --net flag
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #5993 from LK4D4/fix_race_on_stop_#5154

Add synchronization between jobs and server closing
Merge pull request #6008 from rhatdan/umount

We need to umount /var/lib/docker when the daemon exits.
Update ip test to parse new output
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #5998 from crosbymichael/iptables-wait

Add wait flag to iptables
Merge pull request #6015 from crosbymichael/update-test-flags

Update integration tests with --net flag
Add specific branch of jpetazzo's busybox to clone from so we can assume nice things about it

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Simplify "adduser" in the Dockerfile to use the more scripting-friendly "useradd"

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #6019 from tianon/update-dockerfile-busybox

Add specific branch of @jpetazzo's busybox to clone from so we can assume nice things about it