fix docker storage driver

Change-Id: If978411933909a760e92e394abf41c27ed3ba5c2
diff --git a/init.yml b/init.yml
index 0ed24ef..d1b5c9f 100644
--- a/init.yml
+++ b/init.yml
@@ -299,6 +299,7 @@
       options:
         bip: 10.99.0.1/16
         fixed-cidr: 10.99.0.1/17
+        storage-driver: overlay2
     client:
       registry:
         target_registry: apt:5000