Do not use pointer for Mutex

What does this MR do?

This MR is meant to fix a issue discussed in MR !434 (merged): !434 (comment 522916031).

The Gitlab struct does use a pointer to a sync.RWMutex. I refactored the code to change that and also added a test to for the GetDomain() method where the mutex is primarily used to verify that the change works.

TODO

• Added the Changelog (e.g. Changelog: feature) trailer to the commits that need to be included in the changelog
• Added unit tests (if required)
• Added acceptance tests (if required)
• Documentation (if required)
• Code review guidelines
• Go Style guidelines

Closes #555 (closed)