test_nova.sh uses old cirros images

Update test_nova.sh to download Cirros 0.3.1 for config_drive support,
and for consistency with the other Cirros image downloads within this
module.

Closes-Bug: #1275746
(cherry picked from commit eec8af5a745a28db1bcccb51f5cff22ef88dee42)

Change-Id: I2bd462c03d81caa493410e8c7ae583aa71416993