Tweak Geo registry URL configuration

What does this MR do?

Updating Registry related configuration in Geo and other tweaks:

• Moved geo_primary_registry_url to common and documented as required when Registry is enabled to allow full control but default to the same.
• Updated the Geo playbook to convert gitlab_values into a dict and set values directly instead of regex_replace.
• Removed geo_secondary_registry_url as it's no longer required.
• Updated failover script to remove primaryApiUrl from gitlab_values.

#747 (comment 1556899393)

Related issues

Closes Geo + CNH: mishandling of global.geo.registry.r... (#747 - closed)

Author's checklist

When ready for review, the Author applies the workflowready for review label and mention @gl-quality/get-maintainers:

• Merge request:
  • Corresponding Issue raised and reviewed by the GET maintainers team.
  • Merge Request Title and Description are up-to-date, accurate, and descriptive
  • MR targeting the appropriate branch
  • MR has a green pipeline
  • MR has no new security alerts in the widget from the Secret Detection and IaC Scan (SAST) jobs.
• Code:
  • Check the area changed works as expected. Consider testing it in different environment sizes (1k,3k,10k,etc.).
    • 3k CNH Geo
    • Geo Omnibus
  • Documentation created/updated in the same MR.
  • If this MR adds an optional configuration - check that all permutations continue to work.