tools: Expose VmGracefulShutdownTimeout option to engine-config

Change-Id: I0a5f7ebf61a6e5f9524bf15f97e2b4de5ed4c314
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1111253
Signed-off-by: Martin Betak <mbetak@redhat.com>