Use instance specific HDFS caches.

In !118 (merged), we declared an instance specific HDFS cache for platform_eng. We did this because we don't want artifacts from one instance to clash with others.

In this MR we do the same for all other instances. We also remove the global HDFS cache since while debugging on !118 (merged) we found that the default_artifact_caches is additive, and not overridable, which can cause confusion when used jointly with a instance specific cache.