Allow no body PUT to create custom resource class in placement API

A new microversion of the placement API will be created to adjust the PUT /resource_classes/{resource_class} handling to allow the creation of a new or validation of existing resource class in a single request. To put it another way, the PUT request will be made idempotent. The first request will result in a 201 and subsequent requests will result in a 204.

This will replace the currently flawed handling on the same URL which allows a custom resource class to be renamed. This is not functionality we want or need and in fact could be considered dangerous.

For more information see: http://lists.openstack.org/pipermail/openstack-dev/2017-March/114136.html