Changed the discovery_fact to discovery_bootif

*Warning*: You need discovery image 0.5.0+ for this (nightly build). Also make
sure you have `IPAPPEND 2` PXEBoot setting. More info at:

https://github.com/theforeman/ovirt-node-plugin-foreman

This improves the discovery workflow a lot. Recent changes in discovery image
added one extra fact `discovery_bootif` which contains MAC address of the
interface the system booted from.