Based on analysis in If71a09a55d0a36abe0a89bdc9cc39a7e65510737
discussion, CirrOS 0.5.2 is only used by default in unmaintained
branches of devstack and 0.6.1 was never a default in any branch of
devstack. Clean these up to keep the cache reasonable. If jobs
occasionally need different versions, they can still download them.
Change-Id: Id15d0d19ee29a90e56a052d66d3b7d0c90e88e54
ce57525385