Use preemptible instances

tests/cloud_playbooks/create-gce.yml

@@ -24,6 +24,7 @@
         instance_names: "{{instance_names}}"
         machine_type: "{{ cloud_machine_type }}"
         image: "{{ cloud_image }}"
+        preemptible: yes
         service_account_email: "{{ gce_service_account_email }}"
         pem_file: "{{ gce_pem_file | default(omit)}}"
         credentials_file: "{{gce_credentials_file | default(omit)}}"