split PloopTemplate class

PloopTemplate does too much work. We can split all this work
into 3 pieces:

* How VE with existing disk is created
* How to copy disk from cache to specified location
* How to get image from glance

So introduce new classes:

DiskCacheTemplate, which knows, how to create VE.
LZRWCacheTemplate, which can unpack ploop, compressed with prlcompress.
PloopTemplate, which can get image in ploop format from glance.
QemuTemplate, which cah get image in foreign format.
add support of images in form of compressed ploops

Add new type of images: ploops, compressed with prlcompress.
Creating an instance from such image will be much faster,
because it doesn't require any conversions before storing it
to cache.