replace use of cirros 0.3.0 with 0.3.1

cirros 0.3.1 images have several significant improvements over 0.3.0, including:
 * newer kernel (3.2.0-25.40)
 * support for more filesystems (vfat, iso9660)
 * support for configdrive (config-drive-v2) as a datasource
 * acpi reboot/shutdown
 * curl with https support (previously there was no way to get https urls)

Change-Id: Idd7bef0cebb941f77d1cead9dc990fa0b03ff46b
Merge "replace use of cirros 0.3.0 with 0.3.1"