webos-image: Add VIRTUAL-RUNTIME_webos-compositor

Allow other layers to replace luna-sysmgr as the compositor of choice

Open-webOS-DCO-1.0-Signed-off-by: Eric Blade <eric.blade@palm.com>

Change-Id: Ib03d002483cdad60e2b7bb149f5bebf22c97c59a
Reviewed-on: https://g2g.palm.com/2015
Reviewed-by: Eric Blade <eric.blade@lge.com>
Tested-by: Eric Blade <eric.blade@lge.com>
Reviewed-by: Build Verification
Reviewed-by: Roger Stringer <roger.stringer@lge.com>
Reviewed-by: Iyad Qumei <iyad.qumei@palm.com>
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>