Add auth middleware for custom cloud backend

Custom backends will want to provide their own authentication mechanisms
instead of using the Keystone token or EC2-like systems we have in
place.

This adds a new middleware and paste pipeline for the custom backend
that will skip the normal authentication and queries the backend's
`authenticated(context)` method instead.

Since the backend is connected to the Engine whereas the auth middleware
is run in the API service (which may sit on a separate box and have no
access to the engine config or the custom backend itself), we add a new
RPC call that lets API verify the passed credentials.

Change-Id: I2fc4a19564b1e410adb79bd9266f6b6da07dd6c9
Signed-off-by: Tomas Sedovic <tomas@sedovic.cz>