Fix Workaround for OpenAPI Generator Issue

[telday]

Summary

We had to add a workaround for an issue in OpenAPI generator (and partly in Conjur) which caused the request to be sent with a Content-Type: application/json header by default, even with an empty request body.

In order to fix the issue temporarily I added a body to the /host_factory_tokens DELETE method, which tricks the generator into using the text/plain MIME type instead of application/json. This workaround should be removed as soon as either Conjur is updated to handle a request like this properly, or the OpenAPI generator is fixed.

Here is the relevant change in the API spec. The integration test for revoke_host_token will also have to be updated.