Fix sample config external default doc

The sample config file documented the incorrect default for the
[auth].external value as defined in keystone.common.config.

Change-Id: I6f9454f35eaade5fa35738a9ba3f74a388bf5099
Document running with pdb

There was no documentation in "Developing with Keystone" that
makes it obvious that you can use "tox -e debug" to run with pdb
breakpoints.

Change-Id: I82cc827d076a4d95ab6a2f903ceb0cde34005bc3
Merge "Document running with pdb"
Merge "Fix sample config external default doc"