Introduce AuthFailure EC2 error code

AuthFailure is a valid EC2 error code, use it where appropriate.

Add 'AuthFailure' ec2_code attribute to NotAuthorized exception.

Return AuthFailure during Keystone authentication errors and add unit
tests to cover this. When Keystone returns HTTP error code, use
the same code for EC2 error response.

Implements: blueprint ec2-error-codes

Change-Id: I17306d9a5364e095e787dbf0dbb5b88a899372e3
Merge "Introduce AuthFailure EC2 error code"