Use default cirros image to reap the benefits of infra cache

This patch uses DevStack's default.
The default is now 0.4.0.
This avoids cache miss and need to download from flaky mirror.
See [1] for details.

[1] http://lists.openstack.org/pipermail/openstack-discuss/2020-January/thread.html

Change-Id: Ic4598fa5234fb75da99706649db40c5ab925aa27
1 file changed