Add --device flag to allow additional host devices in container

We add a --device flag which can be used like:

 docker run --device /dev/sda:/dev/xvda:rwm ubuntu /bin/bash

To allow the container to have read write permissions to access the host's /dev/sda via a node named /dev/xvda in the container.

Note: Much of this code was written by Dinesh Subhraveti dineshs@altiscale.com (github: dineshs-altiscale) and so he deserves a ton of credit.

Docker-DCO-1.1-Signed-off-by: Timothy <timothyhobbs@seznam.cz> (github: timthelion)
Merge pull request #6961 from crosbymichael/devices-flag

Add --device flag to allow additional host devices in container