For logs stored in object storage services we want to be able to shard
into different containers. This improves scalability of the object
stores. To do this consistently we prefix all log paths with the first 3
characters of the build uuid. This way we get container names like
log_123, log_abc, and so on for a total of 4096 containers.
This behavior is off by default because users of filesystems stores
without a zuul dashboard may still want periodic/ to be filed into a top
level dir for browseability.
Change-Id: I8e72a6e587edcbdf89b793cad2d7c96c535601e7