Add configurable Customer Portal URL for License Mode omnibus config

What does this MR do and why?

In part of running end-to-end tests per MR (internal only), it is required that we provide a configurable CDot endpoint.

How to set up and validate locally

$ bundle exec rspec spec/gitlab/qa/runtime/omnibus_configurations/license_mode_spec.rb

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.