Template and gdk diff-config improvements

What does this Merge Request do?

This MR does the followng:

• Updated config.__uri to always strip of trailing /
• Updates a few templates to use config.__uri instead of piecing host and port together.
• Updated gdk diff-config
  • To be more robust by ensuring config file exists before attempting diff
  • To now check gitlab-runner-config.toml, registry/config.yml and gitlab/config/database_geo.yml

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.