Refactor template list handling

[ericof]

Cookieplone relies on the templates object available at the cookiecutter.json file on the top level of cookieplone-templates to list the available templates.

Even though every template entry under templates has a path property, cookieplone does not use this setting for anything.

It would be interesting to use that information, allowing a better organization of templates inside the cookieplone-templates.