Yamllint tests fail if yamllint isn't installed

[bdunne]

    Failure/Error: _out, err, _ps = Open3.capture3("yamllint -v")
     
     Errno::ENOENT:
       No such file or directory - yamllint
     # ./app/workers/commit_monitor_handlers/commit_range/rubocop_checker/message_builder.rb:129:in `yamllint_version'
     # ./app/workers/commit_monitor_handlers/commit_range/rubocop_checker/message_builder.rb:45:in `header'
     # ./app/workers/commit_monitor_handlers/commit_range/rubocop_checker/message_builder.rb:59:in `build_comments'
     # ./app/workers/commit_monitor_handlers/commit_range/rubocop_checker/message_builder.rb:13:in `comments'
     # ./spec/workers/commit_monitor_handlers/commit_range/rubocop_checker/message_builder_spec.rb:15:in `block (3 levels) in <top (required)>'
     # ./spec/workers/commit_monitor_handlers/commit_range/rubocop_checker/message_builder_spec.rb:21:in `block (3 levels) in <top (required)>'

• We probably shouldn't be shelling out to yamllint during tests
• We should probably handle missing linters better (don't blow up, maybe just skip the linting)

[miq-bot]

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[bdunne]

Copying in comment from @Fryguy in #392 since these are essentially the same issue...

There are a few approaches to fixing this...not sure what's best

* When the yamllint files don't exist, use a shared file from guides

* When the yamllint files don't exist, assuming linting is disabled and don't lint at all

* Add enable/disable support for individual linters, then fail or fallback when the files don't exist

* Send an email instead of making a comment

Also, should the same rules apply for haml lint and rubocop as well?