media_cache: Demote warning to debug as it may occur often.

The file playback system will now query the media cache and then
the old file functionality. Under normal conditions this will result
in the cache failing to retrieve a file causing a warning message
to get output each time a file is played back.

This change demotes this warning to a debug message.

Change-Id: Ib72246ba300b5cce32774bfb3c26634bfb708624

main/media_cache.c

@@ -248,7 +248,7 @@ int ast_media_cache_retrieve(const char *uri, const char *preferred_file_name,
 	 */
 	bucket_file = ast_bucket_file_retrieve(uri);
 	if (!bucket_file) {
-		ast_log(LOG_WARNING, "Failed to obtain media at '%s'\n", uri);
+		ast_debug(2, "Failed to obtain media at '%s'\n", uri);
 		return -1;
 	}