Use tox -e zuul for layout test

Adjust tox environment, the check_projects_yaml_alphabetized.sh script
only works with gerrit/projects.yaml but not with layout.yaml, so remove
the call.

Change the job to use tox -e zuul.

Change-Id: I70d918ec1de94c6ec9aa0c2c6647aa8c791df303
Merge "Use tox -e zuul for layout test"