Configurable default_product_prefix.

Added options multiproduct.default_product_prefix, that can
be used to set prefix for default product. It can be set 
in trac.ini before env is upgraded to multiproduct or by passing
--default-product-prefix to bloodhound_setup.


git-svn-id: https://svn.apache.org/repos/asf/bloodhound/trunk@1485148 13f79535-47bb-0310-9956-ffa450edef68