Changed image data format in fuel_agent

Made it explicit and clear. User may define as many file
system images as he/she needs. Every image is to have
its own uri, format, container and other metadata. Currently,
it is possible to mix different image formats. For example,
image for a root file system might be an ext4 fs image
while /var image - TAR archive.

Change-Id: Ib957ff62e3613fed1e2826f322a056be7803fa48
Merge "Changed image data format in fuel_agent"