As suggested by Hery in https://github.com/3scale/zync/pull/142#discussion_r264277089

Issue: Without this setting apicast_registry_url set, opening the integration page will raise an error 500.

Right now we need in apicast_registry_url of config/sandbox_proxy.yml a url with a valid json. We shouldn't need that just to run Porta locally in development mode, and less now that we have custom policies.
When that is done, we should be able to remove it from the documentation of zync as well

Developer Note:

Different options of implementation

1. Quick win: we can reuse the internal fake backend API https://github.com/3scale/porta/blob/master/config/routes.rb#L31-L38
But we are coupling different independent features so I do not think it is wise

2. use static defaults. That will change the behaviour of the policies (builtin and custom policies) merger but it is fine, sane defaults.

3. Use apicast staging by default. Not really developer friendly (imagine you have no internet connectivity) or apicast staging endpoint become protected

UPDATE:
As a 4th option and the one chosen, policies simply won't show up in the UI when APIcast is not found. This will prevent integration/edit page from failing although no feedback will be given to the user, this will be implemented in a followup PR.