Add more checks for zuul/layout.yaml

Check that each project (with exception of z/tempest) has a merge-check
template.
Check projects - so far stackforge only - for alphabetical order.

The merge template check is taken from a test that Jeremy shared.

Change-Id: I2bc457ddfe0eca787bb88ceeb919f89d491978d7
Co-Authored-By: Jeremy Stanley <fungi@yuggoth.org>
Merge "Add more checks for zuul/layout.yaml"