Fix etcd3 suite

cluster/gce/config-test.sh

@@ -89,7 +89,7 @@ ENABLE_L7_LOADBALANCING="${KUBE_ENABLE_L7_LOADBALANCING:-glbc}"
 ENABLE_CLUSTER_MONITORING="${KUBE_ENABLE_CLUSTER_MONITORING:-influxdb}"
 
 # Set etcd version (e.g. 3.0.4-migration.1) if you need non-default version.
-TEST_ETCD_VERSION=""
+TEST_ETCD_VERSION="${TEST_ETCD_VERSION:-}"
 
 # Default Log level for all components in test clusters and variables to override it in specific components.
 TEST_CLUSTER_LOG_LEVEL="${TEST_CLUSTER_LOG_LEVEL:---v=4}"