Changes to allow better use of custom group types in IGroupForm extensions

A few changes are required in ckan core to allow better use of custom Group types, and hence IGroupForm extension. 

* `logic.action.get.group_list` should only return Groups of a specified type (defaulting to 'group')
* allow IGroupForm extensions to define their own group controller. Currently `lib.plugins.register_group_plugins` hardcodes the 'group' controller.

I'll open a pull request with changes soon.