Automatic merge from submit-queue

Move the defaults setting of GCI to util.sh

fixes #26291 

This change recovers some of the side effects of
https://github.com/kubernetes/kubernetes/pull/26197, i.e., keeps the defaults of
`NODE_IMAGE` and `NODE_IMAGE_PROJECT` to `MASTER_IMAGE` and
`MASTER_IMAGE_PROJECT`, for backward compatibility. Although it keeps
`OS_DISTRIBUTION` defaulting to `gci`, the default settings of these vars are
moved to `cluster/gce/util.sh` and conditioned on `OS_DISTRIBUTION==gci`.

@euank @roberthbailey Can you review?