Fix version string generation to consider branch names and commit SHAs

• Code
• Specs
• Docs

Test Builds with different version string usecases

1. 9.1.0 : https://dev.gitlab.org/gitlab/omnibus-gitlab/pipelines/47869
2. 9.1.0-ee : https://dev.gitlab.org/gitlab/omnibus-gitlab/pipelines/47870
3. SHA : https://dev.gitlab.org/gitlab/omnibus-gitlab/pipelines/47871
4. Branch : https://dev.gitlab.org/gitlab/omnibus-gitlab/pipelines/47872

Closes #2102 (closed)