fix(user-data): restore default discovery url

Author: Gabriel Monroy

contrib/coreos/user-data

@@ -4,7 +4,7 @@ coreos:
   etcd:
     # generate a new token for each unique cluster from https://discovery.etcd.io/new
     # uncomment the following line and replace it with your discovery URL
-    discovery: https://discovery.etcd.io/c560a81e5972043805d8b430adea1a58
+    # discovery: https://discovery.etcd.io/12345693838asdfasfadf13939923
     addr: $private_ipv4:4001
     peer-addr: $private_ipv4:7001
     # give etcd more time if it's under heavy load - prevent leader election thrashing