Add auth middleware for custom cloud backend

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>