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.

(Backported 6558bdc from master.)

src/campaign_server/campaign_server.cpp

@@ -255,7 +255,7 @@ 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;
 			}