Fix Idempotency of configure_containerd.sh script

Created by: matofeder

"Add option to use a custom container registry in containerd" https://github.com/SovereignCloudStack/k8s-cluster-api-provider/pull/432 feature introduced a script configure_containerd.sh. This script injects containerd registry host and cert files into cluster-template.yaml.

This injection is not an idempotent operation. This may cause the containerd registry host and cert files could be added on each script execution.

Add a check which evaluates whether the containerd file is already present in the cluster-template.yaml and make configure_containerd.sh idempotent.