Allow tasks attributes to be preloaded from other tasks

This is similar to inheritance and can help consolidate many task
properties such as hints and common subtasks.  The name "preload" helps
hint at the general policy of overriding scalars, and extending
lists/maps.

This will be particularly useful to at least consolidate the PW
warehouse hints and pass criteria onto the base warehouse task.

Change-Id: I645147e3ce68436cba7fdff8d824fc3c9ef49542
6 files changed