Allow finalize steps for archives before installer creation

It should be possible to polish the archives before creating
the final installer content.
Introduced 'package_finalize_items' option for configuration
files which can list keys. Each key needs matching
functionality from create_installer.py

First option is a clean up for '/doc' directory to clean up
unnecessary content from there.

Change-Id: Ib67a3c7798d79facc3cd7bb4fbc652285145c1e9
Reviewed-by: Sergio Ahumada <sahumada@blackberry.com>
Reviewed-by: Antti Kokko <antti.kokko@digia.com>