labs: update CirrOS image from 0.3.2 to 0.3.3

Update CirrOS image from 0.3.2 to 0.3.3. The reason for updating now is
that 0.3.3 allows setting the MTU via DHCP. Without that feature, users
would have to lower the MTU manually from the instance VM's VNC console.

Change-Id: Iee7530cb41e2a85af4a939fa004ac9e3546b6339