Bump cirros version to 0.6.3

0.6.3 includes some fixes, let's use it.
devstack already switched to it with [1].

[1] https://review.opendev.org/c/openstack/devstack/+/930647

Related-Bug: #2045549
Change-Id: I6c34e8af701102fb56ea8f02def8d129c4ac63dd
1 file changed