WIP: Unplug stalled endpoints (take 2)

This MR unplugs stalled endpoints from docker network. It possibly is a solution for «API error 500» issue appearing when ci-runner tries to create containers.

This MR differentiates from gitlab-org/gitlab-ci-multi-runner!337 in that that it's based on the new docker API, because stalled network endpoints can be deleted only with Force: true flag in docker.NetworkConnectionOptions. The main cause is that by the moment when we are deleting the endpoint, the container does not exist already, so the endpoint could not be deleted without Force.

All tests have been passed, commits were rebased. (fixing merge conflicts)

Fixes #1642 (closed)