Remove etcd directory in uninstall

When we install kuryr-libnetwork, there may be some exception
in install etcd data store, like network timeout, IO error.
Then we may met file exists error reinstall.

We can clean up the etcd directory in unstack script.

Change-Id: I77dd701dcda7866b2d09dd9b334a64df21f5762c
Closes-Bug: #1611187

devstack/plugin.sh

@@ -138,6 +138,7 @@ if is_service_enabled kuryr-libnetwork; then
     if [[ "$1" == "unstack" ]]; then
         stop_process kuryr
         stop_process etcd-server
+        rm -rf $DEST/etcd/
         stop_process docker-engine
     fi
 fi