Use latest stable tag in the repo as fallback for stable branches

What does this MR do?

Fallback to latest available stable tag if none is found in current version series

Testing

1. Checkout 15-11-stable.
2. Set variables CI_COMMIT_BRANCH to 15-11-stable and CI_PROJECT_NAME to omnibus-gitlab.
3. Ensure you don't have build_facts directory present
4. Run bundle exec rake build:generate_facts and see that it fails
5. Cherry-pick this commit to the branch
6. Run the rake task again. Check that build_facts/latest_stable_tag file says 15.10.3+ee.0
7. Remove the build_facts directory.
8. Checkout 15-9-stable.
9. Reset the variable CI_COMMIT_BRANCH to 15-9-stable
10. Run the rake task again. Check that build_facts/latest_stable_tag file says 15.9.4+ee.0

Related issues

Closes #7759 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• Latest Merge Result pipeline is green.
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow.

For GitLab team members

If you don't have access to this, the reviewer should trigger these jobs for you during the review process.

• The manual Trigger:ee-package jobs have a green pipeline running against latest commit.
• If config/software or config/patches directories are changed, make sure the build-package-on-all-os job within the Trigger:ee-package downstream pipeline succeeded.
• If you are changing anything SSL related, then the Trigger:package:fips manual job within the Trigger:ee-package downstream pipeline must succeed.
• If CI configuration is changed, the branch must be pushed to dev.gitlab.org to confirm regular branch builds aren't broken.

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes.
• Documentation created/updated.
• Tests added.
• Integration tests added to GitLab QA.
• Equivalent MR/issue for the GitLab Chart opened.
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.