Ironic: Configdrive extra data

This spec proposes adding support for operators to inject extra data into the configuration drive for Ironic.

perators of baremetal nodes sometimes need to pass extra information about their machines to get their instances configured correctly.

A real use case example is passing information about the network interfaces of the baremetal machine and consistent naming them accordingly. Since the order in which network devices are named in Linux is based on the order in which they were found by the kernel as it booted. This type of configuration is already supported by os-net-config[1] and can be used with heat to do the post-instance configuration of the baremetal node [2].

[1] https://review.openstack.org/#/c/147619/
[2] https://github.com/rdo-management/instack-undercloud/blob/master/heat-templates/ironic-deployment-os-net-config.yaml