Raise Not Implemented error from V2 diagnostics API

In case any driver does not implement functionality of providing VM
diagnostics, HTTPNotImplemented error will be raised from driver.
Nova V2 API should catch the same and pass it with proper error message
for better debugging.

This patch catch & raise  HTTPNotImplemented exception for
V2 "os-server-diagnostics" API.

Adding UT also for the same.

Change-Id: I485d3487d01eba07e3c34f6ef9b5c707af54ebb7
Merge "Raise Not Implemented error from V2 diagnostics API"