Make node_power_cycle more robust.

We want to make sure the machines always pxe, so let's set that
explicitly via ipmi before rebooting - the blades we have in our dev
environment seem to have some issues around settings this permanently in
the BIOS.

Additionally, power_cycle will now still power on a node if it was
previously off.
Merge pull request #245 from zenhack/ipmi-bootdev

Make node_power_cycle more robust.