install-guide: do not use tempdir to download cirros image
install-guide: do not use tempdir to download cirros image

Change-Id: I560450f9d72a5236910e5cd4aedb191c61de7a2b
Implements: blueprint installguide-liberty

openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals