Gitaly service test setup helpers

To break dependency on the global config.Config value in this merge request the gitaly service setup code refactored to generate a distinct config instead of sharing a single one.
It also makes our tests more real as we are moving from 1 gitaly with n storages to n gitalies with 1 storage (or more if needed).
With the new approach we remove unused anymore test setup code that was changing state internally and set some weird configuration.
It also fixes some wrong written tests.

Part of: #2699 (closed)