fix code because of changed Instance.system_metadata format

Now Instance.system_metadata has straightforward format:
{u'image_base_image_ref': u'0178ec4c-3835-4a98-afde-3706d61d58f9',
 u'image_container_format': u'bare',
 u'image_disk_format': u'cploop',
 u'image_min_disk': u'1',
 u'image_min_ram': u'256',
...

So update the code accordingly.
update code because of renamed get_root_heler

Nova developers renamed get_root_helper to _get_root_helper,
and it has become 'private'. But we have to use it, because
nova utils doesn't offer some 'subprocess.Popen' analogue.
don't rely on get_default_vm_folder

get_default_vm_folder returns default directory,
where VMs are being creater. But if a user hasn't set
this property it returns empty string. So use default directory
'/var/parallels' in this case.
fix arguments of ComputeDriver methods

Some method arguments was changed in icehouse release,
so fix them in this driver.
update unittests because of changes in the driver

Update unittests, because driver has been update to
be compatible with icehouse release.