autodeploy tagging should tag release-tools

As the first step toward the single pipeline &154, we want to clearly identify the deployment pipeline in release-tools.

Implementation plan

• rake auto_deploy:tag will be extended to support check mode (probably already supported using dry-run mode)
  • when changes are detected in omnibus or CNG:
    • we will not push the new versions on those repositories,
    • we will tag release-tools with the same version used by release-metadata "#{branch.version.to_minor}.#{branch.tag_timestamp}"
• a new pipeline will be added, it will only run on release-tools tags
  • it will run rake auto_deploy:tag
  • instead of generating the version from the current auto_deploy branch and the timestamp, it will derive that information from the tag name