Add option to use custom container registry in containerd

Created by: matofeder

This PR adds the following:

• Introduces containerd_registry_files variable that allows SCS KaaS users to pass containerd registry hosts config files that configure containerd
• Sets the default value of the containerd_registry_files variable (file terraform/files/containerd/docker.io) that instructs containerd to use registry.scs.community container registry instance as a public mirror of DockerHub
• Script terraform/files/bin/configure_containerd.sh that injects containerd registry host and cert files into cluster-template.yaml. This allows specifying extra files to be passed to user_data upon creation of cluster nodes
• Related docs page doc/usage/containter-registry-configuration.md

Closes #417 (closed)