insert upgrade's /site-packages into sys.path, not append

Put the upgrade image's /site-packages at the head of the line, so
updated ovirt.node.* modules from there get picked up first.
Particularly important for migration and upgrade modules.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1053430
Change-Id: I277245d3aa12cbc336786703e9db0be0224bc072
Signed-off-by: Ryan Barry <rbarry@redhat.com>