Set boot device to PXE when deploying

When deploying a node using the PXE driver, we set the boot device to
'pxe' (both ipminative and ipmitool understand it), and set it as persistent,
so after the deployment, the node boots using the PXE selected kernel

Change-Id: I1d25023f5a0bd46a714395367ceb2ee60a680cff
Closes-Bug: #1264596
Merge "Set boot device to PXE when deploying"