core: Introducing HttpImageTaskVDSCommand

This patch adds HttpImageTaskVDSCommand to contain common logic related
to http image related operations. The related logic was extracted to it
from UploadStreamVDSCommand.

Change-Id: Idc9284e62f75cc4958909bcb03c9b4fa8e30d7d1
Signed-off-by: Liron Aravot <laravot@redhat.com>