Automated Installer Wipe Disk and custom layout functionality

With push of :
    7057944 CUD/AI needs a way to be able to wipe existing layout
        on disk before applying a new layout.

The interpretation of Disk keyword attribute "whole_disk" has changed. This change provides the functionality to wipe a disk and give it a custom layout, ignoring the current contents of the disk.

Prior to this change if a Disk had "whole_disk" set to true it could not contain any partition/slice children. If the disk was assigned to the root pool, the disk was wiped and a default layout was applied to the disk, if assigned to a datapool, then the whole disk name is used as the device when creating the datapool, again effectively wiping the contents.

With this integration, if "whole_disk" is set to true and the Disk specification contains Partition/Slice children, the disk will be wiped and the layout of the disk will be what's specified in the manifest.

If "whole_disk" is set to False, the functionality has not changed, in that AI will attempt to merge what currently exists on the disk with the Partition(s)/Slice(s) specified in the manifest.