This gives us better linting of YAML files that just opening them.
This would have detected the duplicate keys in
I34e27d821fbefe274e7b007f37b0bd34db2e1d26.
The .yamllint is taken from zuul-jobs where it is also used as a
fairly sane set of default rules.
A few minor newline fixes are added.
Change-Id: I96d6644ae24f7deb84fa50fefbda0f0d33e0e009
