Revert "Revert "init: fix S3 ARN parsing aws#617""

This reverts commit 760bfed.

init/fluent_bit_init_process.go

@@ -195,17 +195,20 @@ func processConfigFile(path string) {
 	}
 }
 
-func getS3ConfigFile(arn string) string {
+func getS3ConfigFile(userInput string) string {
 	// Preparation for downloading S3 config files
 	if !s3ClientCreated {
 		createS3Client()
 	}
 
 	// e.g. "arn:aws:s3:::user-bucket/s3_parser.conf"
-	arnBucketFile := arn[13:]
-	bucketAndFile := strings.SplitN(arnBucketFile, "/", 2)
+	s3ARN, err := arn.Parse(userInput)
+	if err != nil {
+		logrus.Fatalf("[FluentBit Init Process] Could not parse arn: %s\n", userInput)
+	}
+	bucketAndFile := strings.SplitN(s3ARN.Resource, "/", 2)
 	if len(bucketAndFile) != 2 {
-		logrus.Fatalf("[FluentBit Init Process] Unrecognizable arn: %s\n", arn)
+		logrus.Fatalf("[FluentBit Init Process] Could not parse arn: %s\n", userInput)
 	}
 
 	bucketName := bucketAndFile[0]