Add element for hardware discovery ramdisk for ironic-discoverd

ironic-discoverd [1] is a StackForge project for conducting hardware properties
discovery via booting a special discovery ramdisk and interrogating hardware
from within it. It aims to be one of the official means of hardware properties
discovery for Ironic in Kilo release [2].

The ramdisk collects hardware information from the machine
it's booted on and posts it to the URL provided via
kernel argument 'discoverd_callback_url'.

[1] https://pypi.python.org/pypi/ironic-discoverd
[2] https://review.openstack.org/#/c/135605/

Co-Authored-By: Lucas Alvares Gomes <lucasagomes@gmail.com>
Change-Id: Ic81fe8b3bd0884971bb522b48658c7ee538a31f2
Merge "Add element for hardware discovery ramdisk for ironic-discoverd"