Allow / as source of -v

We discussed this at the docker plumbers meetup and for tools and
working on the system for things like boot2docker and coreos this is
needed.  You can already bypass this check so we felt it is ok to start
allowing this feature.
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
Remove api test for bind mount /
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
Merge pull request #6701 from crosbymichael/mount-root

Allow / as source of -v