Repackage and consolidate controller helpers and replacement tests

NOTE: This is part of refactoring that is listed in #72 (closed)

This MR contains two sets functional and conventional changes.

The conventional changes, that do not change the behavior of the controller are:

1. Moving helpers into a dedicated package
2. Merge gitlab_test and gitlab packages

The functional changes, that introduce changes to the behavior of the controller are:

1. Load Operator settings from environment variables, including ServiceAccount names (related to #69 (closed)).

Closes #69 (closed)