Add ability to set default scratch size for containers/UVM and re-use…

… LCOW container scratch

* Add containerd.toml options to be able to set the default scratch space size for containers and
the UVMs scratch size for WCOW.

* Add containerd.toml option to be able to specify that we'd like to share the sandbox containers
scratch space for workload containers for LCOW.

* Evaluate symlinks for the sandbox.vhd in the scratch layer as that's what is expected to be
set up by the LCOW snapshotter instead of an absolute path to a previous containers sandbox.vhd.

Signed-off-by: Daniel Canter <[email protected]>