Rework artifact cache config.

Now it's all in pants.ini and the confusing flags are deprecated.

Auditors: pl
Implement rebase more carefully.

Simply deleting lines referencing java home was obviously broken,
as it didn't modify the "items: " line appropriately...

Also fix a bug related to artifact caching of analysis file.

Auditors: pl
Enable the two basic artifact cache flags as overrides.

Auditors: pl