Move PXE instance level parameters to instance_info

The commit 7735ad404bb2849f5be738c2f8a536b1f817c6b7 introduced a
new instance_info field to the Nodes so that all the instance level
information should live there and not mixed in the Node.driver_info
field. This patch is moving all the PXE instance level parameters from
the Node.driver_info to the Node.instance_info.

As all the data in the Node.instance_info is related to the instance,
this field will be completed erased as part of the node's tear down
process. For that reason the _removes_internal_attrs() function from
the PXE driver was removed.

These fields have changed from driver_info to instance_info and
their driver namespace ('pxe_') was removed because they are now
instance-specific parameters:

  'pxe_image_source' -> 'image_source'
  'pxe_root_gb' -> 'root_gb'
  'pxe_swap_mb' -> 'swap_mb'
  'pxe_ephemeral_gb' -> 'ephemeral_gb'
  'pxe_ephemeral_format' -> 'ephemeral_format'
  'pxe_preserve_ephemeral' -> 'preserve_ephemeral'

DocImpact

Implements: blueprint add-node-instance-info
Closes-Bug: #1320090
Change-Id: I608d2cdc6170d0bed826e532dedd120dd32a3b7e
Merge "Move PXE instance level parameters to instance_info"