Remove config.Config from the praefect package

The package internal/praefect contains a source code files that are using config.Config global variable. This MR replaces it with the locally defined alternative. The multi-gitaly simulation with multiple storages on the single instance replaced with actual multiple instances of the gitaly service that run independently.
The global RubyServer variable was removed as well as there is no use for it, once required we could go with the approach used in other places by defining it locally and creating a set of sub-tests that depends on it.
The test setup now looks more tidy and readable and helps better understand the purpose of the test.
The boilerplate code like newReplicationService function removed and replaced with testserver.RunGitalyServer.

Part of: #2699 (closed)