Http Sever: Add power off API

While the shutdown API attempts to do a clean power off, it sometimes fails and halt the shutdown process.
It is sometimes desireable to to a power off, which is force, unsafe shuting down of the system.

This adds a power off functionality to the API.

to test do:
http://localhost:8000/os/poweroff

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Amnon Heiman <amnon@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>