updated startup-script for instances and using latest centos 7 image

gce-properties.yml

diff --git a/gce-properties.yml b/gce-properties.yml
index 2b8c628..ef1cd09 100644
--- a/gce-properties.yml
+++ b/gce-properties.yml
@@ -1,11 +1,11 @@
 ---
 credentials_file: ~/.apigee-secure/default-service-account.json
-machine_type: 'n1-highmem-8'
-#machine_type: 'n1-standard-8'
+#machine_type: 'n1-highmem-8'
+machine_type: 'n1-standard-8'
 #machine_type: 'n1-standard-4'
 #machine_type: 'n1-standard-2'
-image: 'centos-7-v20180104'
-#image: 'centos-7'
+#image: 'centos-7-v20180104'
+image: 'centos-7'
 #image: 'centos-7-v20170918'
 #image: 'centos-7-v20160511'
 #image: 'centos-6-v20170918'