Convert qcow2 image to raw format when deploy

In iscsi deployment, currently the image are converted to raw format
in advance and then copied to the compute node later using DD command.

This patch supports convert on the fly, that means the image is converted
when copying the image to the compute node.

Change-Id: Iae8942c3f4d7c296966fe4325523034dd47e5556
Related-Bug: #1382164
Merge "Convert qcow2 image to raw format when deploy"