updated to use current centos 7 image

gce-properties.yml

diff --git a/gce-properties.yml b/gce-properties.yml
index 947d412..8a7fd4d 100644
--- a/gce-properties.yml
+++ b/gce-properties.yml
@@ -1,7 +1,8 @@
 ---
 credentials_file: ~/.apigee-secure/default-service-account.json
 machine_type: 'n1-standard-1'
-image: 'centos-7-v20170918'
+image: 'centos-7'
+#image: 'centos-7-v20170918'
 #image: 'centos-7-v20160511'
 #image: 'centos-6-v20170918'
 zone: 'us-east1-b'