Make vendor methods discoverable via the Ironic API

The @passthru and @driver_passthru decorators were extended to support
adding a description about the method being decorated which will also be
returned by the API. That said two new endpoints to the API that returns
the available methods for the given driver or node.

* GET /v1/drivers/<driver name>/vendor_passthru/methods

* GET /v1/nodes/<node uuid>/vendor_passthru/methods

Implements: blueprint extended-vendor-passthru
Change-Id: Idac6a433da1a9ae170dbe3d8ceb9e4c76fcdb1be
Merge "Make vendor methods discoverable via the Ironic API"