Versioning for the Ceph API

The main goal for the Ceph RESTful API is to provide a stable interface, which is done by versioning.

To achieve a stable interface, the Ceph API is built on the following principles:

A mandatory explicit default version for all endpoints to avoid implicit defaults.
Fine-grain change control per-endpoint.
- The expected version from a specific endpoint is stated in the HTTP header.
  
  Syntax
```
Accept: application/vnd.ceph.api.vMAJOR.MINOR+json
```
  Example
```
Accept: application/vnd.ceph.api.v1.0+json
```
  If the current Ceph API server is not able to address that specific version, a 415 - Unsupported Media Type response will be returned.
Using semantic versioning.
- Major changes are backwards incompatible. Changes might result in non-additive changes to the request, and to the response formats for a specific endpoint.
- Minor changes are backwards and forwards compatible. Changes consist of additive changes to the request or response formats for a specific endpoint.