update timeout flags for etcd > 2.0

TUNING.md

@@ -23,7 +23,7 @@ the heartbeat interval.
 With our etcd package, you can set these values in
 `/etc/default/etcd` (values are milliseconds):
 
-    DAEMON_OPTS="--peer-heartbeat-interval=1000 --peer-election-timeout=3000"
+    DAEMON_OPTS="--heartbeat-interval=1000 --election-timeout=3000"
 
 Increasing the etcd timeouts causes a related increase in the time
 required to reach consensus and elect a new etcd master in case of