lambda-promtail: add terraform variable for s3 bucket notification filter prefix

[milenpankov-bosch]

lambda-promtail terraform implementation uses a hardcoded value of "AWSLogs/" for filter_prefix in the lambda_function of the aws_s3_bucket_notification resource. This causes an issue with the lambda s3 trigger when using a prefix in the ALB S3 URI and the trigger doesn't work.
The filter prefix should be exposed as terraform variable in the same way as the filter suffix.