Fix layout-checks.py to work in virtualenv

Fix the python interpreter used by layout-checks.py so that the "tox -e
zuul" target works correctly on a local system.

Add the PyYAML dependency needed by layout-checks.py to the tox
configuration for the zuul target.

Add the zuul target to the default list for tox.

Change-Id: Ic4db9fc3bb236d36365abfe966eb9e7769896011