Fix a few places in vnode_setup.in that were stopping plab slivers from
setting up. First chunk: we don't want to skip straight to reboot if the node is plab. Second chunk: we need to run both plabnode alloc and remote vnodesetup on plab nodes. Third chunk: we have to ssh to the vnode, not the pnode, on plab nodes, to make sure we make it to the correct sliver. Not sure how this last affects jails, so I special-cased it for plab.