Add APT pinning configuration for debian in installation docs

Merged Tomasz Maczukin requested to merge documentation/apt-pinning-for-debian-package into master

What does this MR do?

Since Debian Stretch debian repositories have a native gitlab-ci-multi-runner package. And the official repository has the higher priority.

This MR adds the APT pinning configuration step to installation documentation which should prioritize our repository.

Inspired by @caldwell in