60819 yamllint gitlabci ee

What does this MR do?

This merge request adds yamllint (https://yamllint.readthedocs.io) compliance to the ./gitlab directory (and therefore all of the running of .gitlab-ci.yml inside the project.

I have added a custom .yamllint to the project root to make it easier to running yamllint whilst ignoring some aspects of the standards, this could be extended as required for the coding-style of the project with regards to yaml.

Supersedes https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27576

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team