Use onosproject/onos:latest

Change-Id: I96e631918d67c1d2bf46e4edfbd20c3091f1b30b
diff --git a/roles/onos-vm-install/defaults/main.yml b/roles/onos-vm-install/defaults/main.yml
index 68d97bb..4516334 100644
--- a/roles/onos-vm-install/defaults/main.yml
+++ b/roles/onos-vm-install/defaults/main.yml
@@ -3,6 +3,7 @@
 
 trust_store_pw: 222222
 
-onos_docker_image: "onosproject/onos:1.6"
+# ONOS 1.7 not tagged yet, but latest is 1.7
+onos_docker_image: "onosproject/onos:latest"