Configure Gitaly calls to GitLab Shell to go through Internal Load Balancer when applicable (Omnibus)

Closes https://gitlab.com/gitlab-org/quality/gitlab-environment-toolkit/-/issues/152

MR adjusts Gitaly's callback requests(step 6) to GitLab Shell to now go through the Internal Load Balancer for Omnibus environments when GitLab Rails and HAProxy Internal nodes are present. For other types of setups such as Cloud Hybrids or ones that don't have an internal load balancer the main URL is continued to be used for now.

Support is also added for users to use a custom URL for Gitaly callbacks directly instead via the Ansible gitaly_callback_internal_api_url setting.