clean up context on build completion & add test

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
hack/make: fix test-unit target to propogate TESTFLAGS properly

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Merge pull request #7765 from erikh/test_unit_testflags

hack/make: fix test-unit target to propogate TESTFLAGS properly
Merge pull request #7842 from unclejack/build_delete_context

clean up context on build completion & add test
Set container_uuid environment

If you are running with systemd as init 1 and specify the container_uuid environment
variable, systemd will create /etc/machine-id with the correct data.

Then on the host you can setup journald to watch the container.

systemd only allows 32 chars in the UUID stored in /etc/machine-id

Then we can later apply a different patch to setup journald to watch
containers from the host and log all syslog/stdout/stderr data together
int the hosts journal.

The following link explains what systemd expects to be setup.

http://www.freedesktop.org/wiki/Software/systemd/ContainerInterface/

This should replace

https://github.com/docker/docker/issues/3506

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)