Don't chown files to root when running inside a privileged container

This is a noop when the container is privileged.

[#83806940]

Signed-off-by: Chris Brown <cbrown@pivotal.io>
Add test for garden-inside-garden nesting

 - Only runs if GARDEN_NESTABLE_TEST_ROOTFS is exported as an
 environment variable. Should point to a rootfs which supports
 garden-linux (i.e. Ubuntu 14.04+ with cgroups).
 - Add /run to the bind-mounted directories for fall-back container
 filesystem creation. This is necessary because ubuntu symlinks /var/run
 to /run, and without this step /run is not writable.

 [#83806940]

Signed-off-by: Chris Brown <cbrown@pivotal.io>