public.json: Add drop.exclusivity enum

Specs for the enum property [1,2].

[1]: https://github.com/swagger-api/swagger-spec/blob/master/versions/2.0.md#schema-object-
[2]: http://json-schema.org/latest/json-schema-validation.html#anchor76
public.json: Drop the empty basePath

The online validator [1] was complaining:

  [
    {
      "level": "error",
      "domain": "validation",
      "keyword": "pattern",
      "message": "ECMA 262 regex \"^/\" does not match input string \"\"",
      "schema": {
        "loadingURI": "#",
        "pointer": "/properties/basePath"
      },
      "instance": {
        "pointer": "/basePath"
      }
    }
  ]

The specs say [2]:

  If it is not included, the API is served directly under the host.

See also 1580e8a1 (website.json: Initial website specs as Swagger 2.0
JSON, 2014-11-11), which gives more backround for the lack of a
basePath.

After this change, the current specs validate with no errors.

[1]: http://online.swagger.io/validator/debug?url=....
[2]: https://github.com/swagger-api/swagger-spec/blob/master/versions/2.0.md#fixed-fields