Implement power state query, and `on?` helper, for IPMI

This implements the next IPMI operation, testing the remote power state, to
determine if the machine is on or off.  This is a trivial read-only query.

Signed-off-by: Daniel Pittman <daniel@rimspace.net>
Implement "boot from this device once" IPMI support

This adds support for setting a (temporary) boot device via IPMI; the most
likely use-case for Razor is to set it to `pxe` on a node as part of a higher
reliability infrastructure.  (eg: boot local by default, override to PXE on
demand only. :)

Signed-off-by: Daniel Pittman <daniel@rimspace.net>