ci(yamllint): add rule empty-values & use new yaml-files setting

[myii]

• Semi-automated using feat(yamllint): add rule empty-values & use new yaml-files setting myii/ssf-formula#27
• Fix (or ignore) errors shown below:

mysql-formula$ yamllint -s .
./mysql/osfamilymap.yaml
  46:14     error    empty value in block mapping  (empty-values)
  66:14     error    empty value in block mapping  (empty-values)
  67:14     error    empty value in block mapping  (empty-values)
  68:16     error    empty value in block mapping  (empty-values)
  69:17     error    empty value in block mapping  (empty-values)
  70:16     error    empty value in block mapping  (empty-values)
  87:7      error    empty value in block mapping  (empty-values)
  216:14    error    empty value in block mapping  (empty-values)
  217:17    error    empty value in block mapping  (empty-values)

./mysql/defaults.yaml
  29:10     error    empty value in block mapping  (empty-values)
  30:11     error    empty value in block mapping  (empty-values)

[myii]

Need to fix before merging:

        yamllint:
          rules:
            empty-values:
              ignore:
                - mysql/defaults.yaml
                - mysql/osfamilymap.yaml

[myii]

@noelmcloughlin Initial cross-reference: saltstack-formulas/template-formula#164.

I'm requesting your review here because I'm not happy with the fixes being applied. The empty values that are currently there translate to null, which I've explicitly set using ~. That doesn't seem right in the context, though. What are these values supposed to be? Or should those entries be commented out?

The defaults.yaml fixes are fine, it's the osfamilymap.yaml values that I'm asking about.

[myii]

Used inline yamllint ignores to get around this issue in the meantime. Transferred this to issue #229. Self-merging alongside all of the other formulas that have already incorporated this.

[saltstack-formulas-travis]

🎉 This PR is included in version 0.52.1 🎉

The release is available on GitHub release

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