Use dd conv=sparse when writing images to nodes

[Ian Pilcher]

When using whole disk images, one generally wants the *virtual* size of the image to be close to the size of the overcloud nodes' physical disks (since no partition/logical volume/filesystem) expansion is done). As an example, one might end up with a 4GB overcloud image (QCOW file) that has a virtual size of 100GB or more.

Ironic uses qemu-img to convert this QCOW file to a "raw" image, which is a *sparse* file. When the image is copied to an overcloud node's disk (via iSCSI), we currently write all 100GB (mainly of zeroes) across the network to the node's disk.

Adding conv=sparse to the dd command makes it skip the "holes" in the image file; it will only the write the portions of the image that actually contain data across the network.

[Ian Pilcher (arequipeno)]

Patch here - https://review.openstack.org/#/c/534442/

[Dmitry Tantsur (divius)]

pike backport: https://review.openstack.org/535307