iLO Node Discovery

Problem description
===================

Today, Ironic does not have the out-of-band node discovery.
It cannot discover the nodes with in a subnet range or for a
specific driver.

Proposal:
=========
This proposal adds the ability to perform out-of-band
node discovery via multicast search for ProLiant nodes.
Multicast search will be faster than broadcast search.

Proposed change
===============

The proposal is to do node discovery via iLO driver.

* ironic driver-vendor-passthru <driver_name> <method>
         [<arg=value> [<arg=value> ...]]

where driver_name=ilo
    method = node_discovery()
    arg=value pairs can be like:
["manufacturer"="hp", "node_model" = ["SL", "DL", "BL"]

It discovers all ProLiant nodes' iLO IP address that matches the node search
identifiers. A deployer can then use the returned node iLO IP address to
create nodes. This feature can be used with Node property discovery
during node create feature to automate node create.