`TRAVIS_WORKER_GCE_PREEMPTIBLE` currently defaults to true, and `OnHostMaintenance` behavior currently defaults to `MIGRATE` (to avoid jobs being aborted on GCE maintenance events). Therefore, if `TRAVIS_WORKER_GCE_PREEMPTIBLE` isn't overridden, instance creations fail with the following error:
`TRAVIS_WORKER_GCE_PREEMPTIBLE` currently defaults to true, and `OnHostMaintenance` behavior currently defaults to `MIGRATE` (to avoid jobs being aborted on GCE maintenance events). Therefore, if `TRAVIS_WORKER_GCE_PREEMPTIBLE` isn't overridden, instance creations fail with the following error:

```
googleapi: Error 400: Invalid value for field 'resource.scheduling.preemptible': 'true'. Scheduling must have preemptible be false when OnHostMaintenance isn't TERMINATE., invalid
```

This PR:
  - makes `TRAVIS_WORKER_GCE_PREEMPTIBLE` default to `false`
  - if `TRAVIS_WORKER_GCE_PREEMPTIBLE` is set to true and `onHostMaintenace` is set to `MIGRATE` (which is the case by default for non-GPU VMs), the latter is overridden by `TERMINATE`.

travis-ci/worker,travis-ci/worker,travis-ci/worker