ERP5: rework frontend instance parameter

This change the format or the (mostly) unused frontend parameter to support requesting more than one frontend and also enable the request of a frontend by default, so that requesting a frontend separately is no longer needed.

The frontend parameter now also supports requesting frontends for specific paths on the ERP5 backend, the example below requests a frontend serving directly a web site, with the necessary rewrite rules:

{
  "frontend": {
    "default": {
      "internal-path": "/erp5/web_site_module/renderjs_runner/"
    }
  }
}

The example below requests a default frontend to the erp5 root, to access the ZMI or erp5_xhtml_style interface and two web sites:

{
  "frontend": {
    "default": {},
    "erp5js": {
      "internal-path": "/erp5/web_site_module/renderjs_runner/"
    },
    "crm": {
      "internal-path": "/erp5/web_site_module/erp5_officejs_support_request_ui/"
    }
  }
}

The example below has an explicit definition of the zope families using zope-partition-dict parameter, because there are no family named "default", no frontend is requested by default:

{
  "zope-partition-dict": {
    "backoffice": {
      "family": "backoffice"
    },
    "web": {
      "family": "web"
    },
    "activities": {
      "family": "activities"
    }
  }
}

Continuing this example, to have frontends for backoffice and web families, the frontend request can specify the families, like it is demonstrated in the example below. In this example, we don't specify an entry for "activities" family, so no frontend will be requested for this family.

{
  "frontend": {
    "backoffice": {
      "zope-family": "backoffice"
    },
    "web": {
      "zope-family": "web",
      "internal-path": "/erp5/web_site_module/web_site/"
    }
  }
  "zope-partition-dict": {
    "backoffice": {
      "family": "backoffice"
    },
    "web": {
      "family": "web"
    },
    "activities": {
      "family": "activities"
    }
  }
}