Image caching support

Creating bootable volumes can be a time consuming task. In many scenarios,
fetching images every time can be highly undesirable, especially if there
are just a few large images used very often.

In some cases, when virtual disk images are used to store the volumes,
using differencing disks can reduce even more the required time as well as
disk space.

This blueprint proposes adding the image caching feature with a similar
workflow as used by Nova.