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:

```js
{
  "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:

```js
{
  "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 is more than one zope
family, no frontend is requested by default:

```js
{
  "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.

```js
{
  "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"
    }
  }
}
```