<broker> enable customizing default app templates

https://trello.com/c/EoO1OFal/200-2-enterprise-admin-can-change-default-app-git-templates
This enables the administrator to configure URLs that should be used as
the default application templates for framework cartridges. This is done
in broker configuration.
<broker unit tests> for config readers

related to parsing app template url config
https://trello.com/c/EoO1OFal/200-2-enterprise-admin-can-change-default-app-git-templates
<broker func tests> for custom default templates

https://trello.com/c/EoO1OFal/200-2-enterprise-admin-can-change-default-app-git-templates