etcd: add support for user specified ports for testing

sample-lnd.conf

@@ -984,6 +984,12 @@ litecoin.node=ltcd
 ; Useful for testing.
 ; db.etcd.embedded=false
 
+; If non zero, LND will use this as client port for the embedded etcd instance.
+; db.etcd.embedded_client_port=1234
+
+; If non zero, LND will use this as peer port for the embedded etcd instance.
+; db.etcd.embedded_peer_port=1235
+
 [bolt]
 ; If true, prevents the database from syncing its freelist to disk. 
 ; db.bolt.nofreelistsync=1