:sparkles: Add unit tests for the openstackclusterstackrelease controller

[chess-knight]

What this PR does / why we need it: Related to #49

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #

Special notes for your reviewer:

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• [ ] squash commits
• [ ] include documentation
• [x] add unit tests