Make it possible to run zuul layout test locally

Add a tox environment to run the zuul layout tests locally.

Modify run-layout.sh to ignore a missing mirror selection script
and assume that it is running with a valid mirror configuration.
This allows the script to be run locally without breaking a
dev environment.

Change-Id: I5b0c94ae51e7f41b8b26b3ef4f78b830031d263e
Merge "Make it possible to run zuul layout test locally"