[netsvc] Use device.manager.Administrator interface for reboot

The write hook to dmctl has been removed.

Tested by causing bootserver to send a reboot command. One way to do
this is to pass bootserver an FVM but not a kernel.

out/default.zircon/tools/bootserver \
  --board_name chromebook-x64 \
  --fvm /out/default/obj/build/images/fvm.sparse.blk \
  -n <node name>

ZX-4038 #comment

Change-Id: Ibb9ea419feb542aa811c60e0d280f68ae2b86144