Document tox instead of run_tests.sh

Change-Id: Ib4619240c27b0dfe861c9297b64e6d52cdde8944
Closes-Bug: 1244605
Documentation on how-to develop Keystone Extensions

This file documents in details how to develop an
extension following the Keystone guidelines.
It also improves the already existing `example` extension
adding some boilerplate code.

bug #1218596

Change-Id: I559aa219f8552c00d326c1b766078ff306e94bb9
Remove obsolete driver test module

Now that all drivers use the abc module to create abstract
base classes, there is no need to test that they raise
NotImplemented by default.

bp abstract-base-class-drivers

Change-Id: Ic2c488c0cee81004291bd355ae475446e5a86aa7
Merge "Document tox instead of run_tests.sh"
Merge "Documentation on how-to develop Keystone Extensions"
Merge "Remove obsolete driver test module"