Cleanup nova exception message conversion

Fix cases where setting the explanation for an HTTP exception
based on a Nova exception was done using str(ANovaException)
rather than using ANovaException.format_message()

Change-Id: Ia8a361fcb088dcf3c17be0d296aee494b9f063d7
Merge "Cleanup nova exception message conversion"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp