template: introduce ImageDownloader classes
template: introduce ImageDownloader classes

Split templates class hierarchy. There is one task, which is
not related directly to creating container from an image:
downloading image from glance to image cache.

So add ImageDownloader base class and move code, which
deals with image download and conversions to his subclasses.

There is one method fetch_to_<smth> method for each type
of image cache, but now there is only one image cache type
in form of tarred and compressed with prlcompress ploop
images. So the only 'fetch_to_lzrw' method is implemented.

CloudServer/pcs-nova-driver,CloudServer/pcs-nova-driver