confine: add node restApi fields: sliver.disk, node.disk_avail, node.disk_max_per_sliver, node.disk_dflt_per_sliver
confine: check sliver rootfs for illegal links before customization
vct: add vct_build_node_base_image, vct_build_sliver_exp_data, and unfinished vct_build_sliver_template