Adding accounting config read/write to admin rest api and cli to set/get/rm them

This is similar to the perm and zone configs.

I'm going to try to consolidate some of the handler cli code next.

Actually, this cluster_id is nonsense. Shawn added it for some other temporary test purpose. Right now there's nothing to put in an accounting config. Just the presence of one signifies accounting should be done on that key prefix. I think we should change this API not to take a file. On CmdGetAcct, we should just print "accounting is enabled for prefix %q".