Fixes a Python3 syntax error using raise

In Python 3 you can no longer use the 'raise Class, *args' syntax when
raising exceptions.

Change-Id: I25781e8b57b06afa1ff6da0104a29d80c2a73ffb
Implements: bp keystone-py3kcompat
revise example extension directory structure

- used the syntax employed by the 'tree' tool, which is not so heavy on
  whitespace, and should render much better as a literal example on the
  web
- merged the 'backends' example into the general example, since having
  at least one backend is common
- flushed out an example 'migrate_repo' directory

Change-Id: Idba9555f93caf2142e0d1becdc1afb0af50b94de
Merge "revise example extension directory structure"
Merge "Fixes a Python3 syntax error using raise"