etcd: add (dev only) disabletls option for etcd

sample-lnd.conf

@@ -968,6 +968,9 @@ litecoin.node=ltcd
 ; Etcd namespace to use.
 ; db.etcd.namespace=lnd
 
+; Whether to disable the use of TLS for etcd.
+; db.etcd.disabletls=false
+
 ; Path to the TLS certificate for etcd RPC.
 ; db.etcd.cert_file=/key/path