Dry template utils and use tmpl engine for rails and workhorse configs

What does this MR do?

This MR is based on !46 (merged), which includes some fixes for gitlab.yaml config. Do not merge before !46 (merged).

It includes the following changes:

• The rails and workhorse configurations are rendered using the template engine. !47 (ada3a514)
• Unit tests using fake go-client are added to test the rendering task. !47 (9b128eda). We may wait for !45 (diffs) to have everything in test_uils
• MakeContext in the test utils now accepts options.
• Unused functions and vars are removed as part of #42 (closed)

Author's Checklist

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Required

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Expected

• Test plan indicating conditions for success has been posted and passes.
• Documentation is created or updated.
• Tests are added.

Related issues

Closes #42 (closed)