Update cirros256 flavor to have some disk

It seems nova has changed defaults on who can create zero-sized disk
instances [1] and now some devstack jobs, like nodepool's, can't
create cirros images using this flavor.  It seems the easiest thing to
do is just to bump it up.

[1] https://review.openstack.org/#/c/603910/

Change-Id: I1172d4775d608568ccbeb27e2975d83add892ea9
1 file changed