Move the 'instance_info' fields to GenericDriverFields

The instance_info parameters are common across all drivers and not
just for PXE drivers.

This patch corrects an issue in nova/virt/ironic/patcher.py by moving
the generation of the instance_info patch to the common base class.
It also adds notes stating the intent to remove the PXE-specific
patching in Kilo.

Closes-Bug: #1353631
Change-Id: I80d2208d41d21821f0da662637418ecd76b5db2b
Merge "Move the 'instance_info' fields to GenericDriverFields"