Tweak the Toolkit to support building Hybrid Cloud Native backend components

The MR adds small tweaks to support building Omnibus components for Hybrid Cloud Native Deployment:

• Adding a default value for gitlab_rails_int_ips if Rails nodes are missing. When building Hybrid environments only GitLab Webservice and GitLab Sidekiq pods are being deployed to the cluster. In GET Rails and Sidekiq nodes should be removed thus we need to add default values for Rails IPs if they're missing to avoid exceptions.
• Adding gitlab_shell['secret_token'] in Rails and Gitaly nodes. It helps to create a secret for GitLab Shell only once and redeploy pods to the cluster. Otherwise we would need to build Omnibus components with GET first, then pull secret_token from gitlab-secrets.json and create a new secret for GitLab shell in the cluster. If we needed to rebuild Omnibus components, we would need to create a new secret for GitLab shell.
  • Additionally it aligns with the current reference architecture docs