[controller] Merge cmd subpackage into main

[astef]

Description

Scope of the change is sds-local-volume-scheduler-extender. My goal is refactoring, not changing the behavior, but a few edge case scenarios has been fixed.

• simplify project structure by merging cmd subpackage into main
• extend sds-local-volume-scheduler-extender/pkg/logger with options to remove repetetive [subMain] logger name prefix
  • this refactoring was reverted, it will be proposed separately
• use one-line error checks (if err := foo(); err != nil {) where possible
• add error logs, where they were missed
• fix error handling code (sometimes we were not exiting after logging the error)
• replace os.Exit(1) with errors, since former doesn't execute defer (may lead to accidental resource leakage in future)
• configure cobra's SilenceErrors = true to avoid printing error twice (once by us and another one by cobra)
• refactor graceful shutdown:
  • configure OS signal listener earlier - in main, in order to react to quick interruptions
  • support SIGINT signal, for shutdown to work from terminal
• specify WriteTimeout for http.Server to avoid indefinite wait for slow clients during shutdown
• provide BaseContext to Runnables managed by the Manager

Why do we need it, and what problem does it solve?

Code becomes cleaner, and a few minor edge-case problems has been fixed.

What is the expected result?

Behavior should not change

Checklist

• Changes were tested in the Kubernetes cluster manually.