Document automated reprovisioning for bare metal

We want to encourage users to reprovision their bare-metal nodes when their config changes, but we don't document how to do it. In the long run, https://github.com/coreos/fedora-coreos-tracker/issues/399 might be the right approach, but in the short term, other mechanisms are required. This is arguably part of https://github.com/coreos/fedora-coreos-docs/issues/117, but I'm filing it separately to distinguish the higher-level workflow issue from the nuts-and-bolts topics in #117.

There are a few possible reprovisioning flows:

• UEFI + PXE: use efibootmgr --bootnext to switch the machine into netboot mode just long enough to reinstall the node. (I haven't actually tried this.)
• PXE generally: set the machine to always netboot, but have the netboot server redirect the boot back to the local disk unless it's been told to reprovision this particular machine. Does Matchbox help here?
• IPMI + virtual CD + ISO: use coreos-installer iso kargs embed to set coreos.inst.install_dev= and related kargs, then push the ISO to the system with IPMI.
• Just run from live ISO / PXE and never install to disk. This requires some other mechanism for OS updates.

Whichever flows we decide to recommend, we should provide step-by-step instructions.

Review issues #117 and #399 first, then validate the proposed UEFI/PXE, Matchbox, IPMI virtual-CD, and live-ISO/PXE reprovisioning flows. Document the recommended flows with step-by-step instructions, including the mentioned efibootmgr and coreos-installer commands; done means users can follow the guidance to reprovision a bare-metal node after configuration changes.