puppet-lint: fail on warnings

[bastelfreak]

It's the current community best practice to actually honour the
puppet-lint warnings and fail during CI. pdk should reflect that as
well.

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[chelnak]

Hey, i'm closing this as this setting is present in spec_helper.

https://github.com/puppetlabs/puppetlabs_spec_helper/blob/01886e8fb8c9470e74ffde39445732ce06209125/lib/puppetlabs_spec_helper/rake_tasks.rb#L156