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.

kvdb/etcd/config.go

@@ -10,6 +10,8 @@ type Config struct {
 
 	EmbeddedPeerPort uint16 `long:"embedded_peer_port" description:"Peer port to use for the embedded instance. Note: use for testing only."`
 
+	EmbeddedLogFile string `long:"embedded_log_file" description:"Optional log file to use for embedded instance logs. note: use for testing only."`
+
 	Host string `long:"host" description:"Etcd database host."`
 
 	User string `long:"user" description:"Etcd database user."`