Change default label of container volumes to shared SELinux Label

Since these will be shared between containers we want to label
them as svirt_sandbox_file_t:s0.  That will allow multiple containers
to write to them.

Currently we are allowing container domains to read/write all content in
/var/lib/docker because of container volumes.  This is a big security hole
in our SELinux story.

This patch will allow us to tighten up the security of docker containers.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
Merge pull request #7660 from rhatdan/selinux-volumes

Change default label of container volumes to shared SELinux Label