prepare_instance() is not called for whole disk images with 'agent' deploy interface

[Shivanand Tendulker]

task.driver.boot.prepare_instance gets called only if the instance image is partition image. It is not getting called when whole disk image is used.

https://github.com/openstack/ironic/blob/master/ironic/drivers/modules/agent.py#L287

This causes failure in ceratin cases wherein driver needs to perform the steps after provisioning images (both partition and whole disk). Ex. iLO driver needs to set boot mode and update secure boot settings as part of prepare_instance() after provisioning the image.
https://github.com/openstack/ironic/blob/master/ironic/drivers/modules/ilo/boot.py#L495-L498

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/499050