Fix EC2 API Fault wrapper

Fault wrapper now returns actual EC2 error code instead of HTTP
status code which was ridiculously incorrect behavior.

Fault:__call__ wasn't covered by any unit tests, so I added one.

Implements: blueprint ec2-error-codes
Change-Id: I41f7f0aa09e2925882a0da941f9ea5d4e694e65c
Merge "Fix EC2 API Fault wrapper"