etcd: enable optional log file for embedded etcd log output

In this commit we add an extra config for enabling logging to an
external file when using embedded etcd. This can be useful when running
integration tests to see more details about etcd related issues.

sample-lnd.conf

@@ -1131,6 +1131,10 @@ litecoin.node=ltcd
 ; If non zero, LND will use this as peer port for the embedded etcd instance.
 ; db.etcd.embedded_peer_port=1235
 
+; If set the embedded etcd instance will log to the specified file. Useful when
+; testing with embedded etcd.
+; db.etcd.embedded_log_file=/path/etcd.log
+
 
 [cluster]