Fix use of Range header in log-pusher.py.

* modules/openstack_project/files/logstash/log-pusher.py: The Range http
header was not properly applied, fix it.

Change-Id: I9ccccd57f7d72853dfbce0c1593737e0f6495724
Reviewed-on: https://review.openstack.org/26668
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins