repmgrd: add parameter "failover_delay"

This parameter is not documented and intended for use during testing.
It should not be used in production.

configfile.h

@@ -240,6 +240,7 @@ typedef struct
 
 	/* undocumented test settings */
 	int			promote_delay;
+	int			failover_delay;
 	char		connection_check_query[MAXLEN];
 } t_configuration_options;