Allow boto to get S3 credentials for backups from environment variables if access key is blank (#2260)

In case that no static AWS credentials are specified, we try to create the boto3 client without explicitly passing static credentials. This way, we can benedit from dynamic credentials in AWS environments (e.g. using EC2 instance roles)

management/backup.py

@@ -519,10 +519,13 @@ def list_target_files(config):
 
 		# connect to the region & bucket
 		try:
-			s3 = boto3.client('s3', \
-				endpoint_url=f'https://{target.hostname}', \
-				aws_access_key_id=config['target_user'], \
-				aws_secret_access_key=config['target_pass'])
+			if config['target_user'] == "" and config['target_pass'] == "":
+				s3 = boto3.client('s3', endpoint_url=f'https://{target.hostname}')
+			else:
+				s3 = boto3.client('s3', \
+					endpoint_url=f'https://{target.hostname}', \
+					aws_access_key_id=config['target_user'], \
+					aws_secret_access_key=config['target_pass'])
 			bucket_objects = s3.list_objects_v2(Bucket=bucket, Prefix=path)['Contents']
 			backup_list = [(key['Key'][len(path):], key['Size']) for key in bucket_objects]
 		except ClientError as e: