campaignd: Write config to disk every ten minutes, not 60 seconds

The 60 seconds value was just intended for internal testing and it
slipped into production by accident.

src/campaign_server/campaign_server.cpp

@@ -256,7 +256,7 @@ void server::run()
 
 			const time_t cur_ts = time(NULL);
 			// Write config to disk every ten minutes.
-			if(cur_ts - last_ts >= 60) {
+			if(cur_ts - last_ts >= 10*60) {
 				write_config();
 				last_ts = cur_ts;
 			}