Provide settings to change containerd's persistent storage

[arnaldo2792]

What I'd like:

Originally posted in #1209

I'd also like to be able to use this storage (ephemeral disks) for the images pulled by the container runtime.

Containerd will store container images in the persistent data storage which is setup with the Root property in /etc/containerd/config.toml. We need to provide a mechanism to allow users to use ephemeral disks as containerd's persistent data storage.

Any alternatives you've considered:
With #1601, users can set up and mount the ephemeral disks attached to the host. However, they can't mount the disks directly to where Root points (/var/lib/containerd) since mounts will only propagate on /mnt in the host. Another problem is that any new mounted filesystem will be labeled with unlabeled_t rather than with local_t, so we need to figure out what's the best way to handle the initial relabel in those filesystems

[arnaldo2792]

It is possible to set up ephemeral disks as the storage for container images, as described in #1991