Add linters check to make sure site-variables is yaml
Add linters check to make sure site-variables is yaml

A YAML syntax error in site-variables was merged recently. Errors in
that file break basically everything, so add a check that the file at
least parses.

Change-Id: I9b2c47a0461275e1d53aabe387f0309719d46b77

openstack-infra/project-config,openstack-infra/project-config