Allow apps to be installed without helm installed if local tiller enabled

What does this MR do?

If the managed_apps_local_tiller FF is enabled, allow Gitlab-managed apps to be installed without Helm installed

backend change only behind a feature flag

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.