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
Adding custom bridge support to network driver.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Adding custom bridge support in docker.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Custom bridge support added to container linking logic.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Updating the docs to reflect the newly added custom bridge support in docker run.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Fixing comments.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Updated docker-run.1.md with custom bridge option.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)