Support updating IPMI credentials from within ramdisk

If ipmi_setup_credentials is set to true in Node.extra, return
desired credentials back to the ramdisk and wait for power management
interface to pass validation.

Unfortunately, due to newly revealed problems, this is reverting
a big part of blueprint returning-to-ramdisk.

Change-Id: I63976d01f265d18c385f6a30f1f2884c58ca5a43
Implements: blueprint setup-ipmi-credentials