don't import anything from nova in utils.py

I want to use it in pcs-glance-tool, so don't import
nova packages.
add _get_boot_disk method to pcs-glance-tool

I want to move it to utils.py, so it's better to have
one method _get_boot_disk instead of _get_ct_boot_disk and
_get_vm_boot_disk.
import prlsdkapi directly

We will never import this module on hosts, which don't
have prlsdkapi installed. So there is no need to import
in in class constructors.
move get_boot_disk function to utils.py
move convert_image function to utils.py
add class CPloopUploader

Add class, whcih can upload compressed ploop to the glance,
and it doesn't depend on interface with glance, so it can be
used both in pcs-glance-tool and in the driver, in snapshot
method.
move CPloopUploasder class to utils.py
remove broken method snapshot

It is more clear to remove broken method snapshot and
then implement it from scratch.
fix system_exc function in utils.py

ret variable name used everywhere as a return code,
so let's use it there instead of 'p'.
add ability to provide root_helper to convert_image

qemu-img needs to read from a ploop device, so we
need to run it as root. Also ploop mount must be run under
root.
implement snapshot method

Add new snapshot method, which supports all disk formats.
Format, which is used by default for snapshots is specified
in 'pcs_snapshot_disk_format' config option.