Addresses part of issue #51487.

This is a big change for testing; any testjobs that do not
set an explicit KUBE_NODE_OS_DISTRIBUTION will have been running
on CVM, but after this PR will start running COS.

CVM is being deprecated, and falls out of support on 2018/10/01.

In addition, bump the patch version of COS from
cos-stable-60-9592-84-0 to cos-stable-60-9592-90-0.