TemporaryImages to inspect image before conversion

TemporaryImages to inspect image before conversion

This intruduces TemporaryImages utility class which enables
download images to temporary file for inspection before they are
used for format conversion. Using this utility, we can get image
information such as virtual size, which is required to implement
image cache, before calling fetch_to_raw() etc., without
downloading the image twice nor changing APIs of image_utils.

Change-Id: I6c2d48bca60e5b9e1615bbe10602d925ae6d75ff
Partially-implements: blueprint image-volume-cache