Fix status section out of order problem

mutiple status.go in go/cmd add status section in vt status server
for system monitoring purpose. However, the problem is they use init
function and may cause potential section out of order problem on
status page if there is another place to also add a new section.

The fix is to introduce a pluggable hook in each status.go so that
whenever another place want to add a new section, it could do so by
assign the hook in its init method.