Make "pgdata" a mandatory configuration file setting

There are some circumstances, e.g. during switchover operations,
where repmgr may need to operate on a data directory while the
server isn't running, in which case there's no way to retrieve
that information.

configfile.h

@@ -53,9 +53,9 @@ typedef struct
     int			node_id;
 	char		node_name[MAXLEN];
 	char		conninfo[MAXLEN];
+	char		pgdata[MAXPGPATH];
 	char		replication_user[NAMEDATALEN];
 	char		pg_bindir[MAXPGPATH];
-	char		pgdata[MAXPGPATH];
 	int			replication_type;
 
 	/* log settings */