Add custom url helpers

:convenience_store: 

I'll have another PR for the cookbook up shortly.

Closes https://github.com/opscode/supermarket/issues/889
I don't think we should be setting any of these here in .env. They all have reasonable defaults in the helper module, and if we have them all here, then those defaults won't be used correctly if we don't also set all of these in Omnibus.

If I'm using Omnibus, under normal circumstances I should just need to set `CHEF_SERVER_URL` and everything else should work.