Allow IPC namespace to be shared between containers or with the host

Some workloads rely on IPC for communications with other processes.  We
would like to split workloads between two container but still allow them
to communicate though shared IPC.

This patch mimics the --net code to allow --ipc=host to not split off
the IPC Namespace.  ipc=container:CONTAINERID to share ipc between containers

If you share IPC between containers, then you need to make sure SELinux labels
match.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
Writeup on what exactly @SvenDowideit? :) On why we'd want this feature from Docker, or on the short summary of our use cases 1 and 2?