Support for full disk images

TripleO only support using flat partitioned images, it means that qcow2, initrd and vmlinuz needs to be provided and uploaded.
But there can be another use case, that is supported in Ironic, and that is the ability to use full disk images, as documented on:

http://docs.openstack.org/project-install-guide/baremetal/draft/configure-integration.html#configure-the-image-service
Currently if you just provide the qcow2 image, the command fails complaining about missing initrd and vmlinuz.

TripleO shall allow to upload just qcow2, detecting that we try to provide a full disk image, and do not setting kernel_id and ramdisk_id on the glance properties for the qcow2 image. This will enable support for full disk images that ironic provides.

In order to generate the full disk images, the "vm" element needs to be used on diskimage-builder elements , instead of relying on "baremetal" one. If customers choose to use full disk images, they would need to use this element (or customized ones) to generate those.

In parallel, there is a refactor of diskimage-builder going on, to be able to compose images with partitions, volumes, etc... This will allow to create images with partitions, volumes, etc... that will fit the customer needs: https://review.openstack.org/#/c/336946/ . However work on TripleO is not depending on that, but will benefit of that features when they land.