use newer 0.3.2 cirros image

This version of cirros has a udhcp daemon that supports dhcp option 121,
which allows passing of metadata static route via dhcp.  Important for
testing using neutron and metadata agent.

Also switch to download from new download.cirros-cloud.net location

Change-Id: Ic7e6d23407a282c4320b915549b76ab7f86381f6