All this changes only impact tests.

Our specs shouldn't depend on whether the tests are run on the master
branch or on a stable release branch, or a tag branch. In the master
branch gitlabVersion resolves to master but on stable branches it will
resolve to a semVer.

The solution proposed here is to:

- Update HelmTemplate.defaults helper to include
global.gitlabVersion:"v42.0.0".
- Update all usages of this helper to merge the values using deep_merge
  instead of merge when customizing values.
- Fix the HelmTemplate.with_defaults(yaml) to override the defaults with
  the passed yaml and not the other way arround.
- Adapt the tests that actually test not passing gitlabVersion to
  not use the HelmTemplate.defaults, but to use only the HelmTemplate.certmanager_issuer
  which is the only default it needs to be tested. For this specific
  test we'll keep it dependent on the branch, but we have a preexisting fetch_default_gitlab_version
  helper method that takes care of it.

Additional cosmetic updates:
- Use deep_merge! instead of deep_merge for consistency
- Rubocop also fixed a couple of identations