Automatic merge from submit-queue (batch tested with PRs 48970, 52497, 51367, 52549, 52541). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>..

Default ABAC to off in GCE (for new clusters).

**What this PR does / why we need it**:
Disables the legacy ABAC authorizer by default on GCE/GKE clusters using kube-up.sh. Existing clusters upgrading to 1.8 will keep their existing configuration.

**Release note**:
```release-note
New GCE or GKE clusters created with `cluster/kube-up.sh` will not enable the legacy ABAC authorizer by default. If you would like to enable the legacy ABAC authorizer, export ENABLE_LEGACY_ABAC=true before running `cluster/kube-up.sh`.
```