Integrate yamllint

[hagau]

A linter for the YAML files would be great.
yamllint is a likely candidate for that task, but has (this is of course my personal opinion) rather stringent and somewhat opinionated defaults. Still, after some experimentation, integration should not be too difficult.

[hagau]

Commit 9827509 adds very rudimentary linting via yamllint.
This obviously needs further work. The next step would be writing the configuration for yamllint, then proper integration by using the python module itself instead of running it in a subprocess can be implemented.