Automatic merge from submit-queue (batch tested with PRs 43546, 43544)

Default to enabling legacy ABAC policy in non-test kube-up.sh environments

Fixes https://github.com/kubernetes/kubernetes/issues/43541

In 1.5, we unconditionally stomped the abac policy file if KUBE_USER was set, and unconditionally used ABAC mode pointing to that file.

In 1.6, unless the user opts out (via `ENABLE_LEGACY_ABAC=false`), we want the same legacy policy included as a fallback to RBAC.

This PR:
* defaults legacy ABAC **on** in normal deployments
* defaults legacy ABAC **on** in upgrade E2Es (ensures combination of ABAC and RBAC works properly for upgraded clusters)
* defaults legacy ABAC **off** in non-upgrade E2Es (ensures e2e tests 1.6+ run with tightened permissions, and that default RBAC roles cover the required core components)

GKE changes to drive the `ENABLE_LEGACY_ABAC` envvar were made by @cjcullen out of band

```release-note
`kube-up.sh` using the `gce` provider enables both RBAC authorization and the permissive legacy ABAC policy that makes all service accounts superusers. To opt out of the permissive ABAC policy, export the environment variable `ENABLE_LEGACY_ABAC=false` before running `cluster/kube-up.sh`.
```