core: Extracting OvfDataUpdater logic to commands

This patch moves the ovf update logic resides within the
OvfDataUpdater to BLL commands, so that it can be executed easily by
other flows as well.

The logic was mainly moved to ProcessOvfUpdateForStoragePoolCommand,
tests were added accordingly.

Change-Id: I6780aedf2d215d08c4bed0ff4f7520c45f753f66
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1138124
Signed-off-by: Liron Aravot <laravot@redhat.com>