Make it possible to run zuul layout test locally
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

noorul/os-project-config,Tesora/tesora-project-config,open-switch/infra_project-config,dongwenjuan/project-config,citrix-openstack/project-config,dongwenjuan/project-config,Tesora/tesora-project-config,openstack-infra/project-config,noorul/os-project-config,osrg/project-config,coolsvap/project-config,anbangr/osci-project-config,openstack-infra/project-config,open-switch/infra_project-config,anbangr/osci-project-config,osrg/project-config,coolsvap/project-config,citrix-openstack/project-config