ansible-playbook: refactoring to better handle new docker registries

[Steven Webster]

Brief Description
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The ansible-playbooks code should be re-factored to better handle the list of supported docker registries. This would make it easier and less error-prone to add new registries to the list.

An example of this is here:

https://review.opendev.org/c/starlingx/ansible-playbooks/+/803499

In which support for the github docker registry is added.

As can be seen, there are multiple tasks which reference the list of registries and their respective urls.

It would be nice if this could be defined once at a higher level.

Severity
--------
Minor: System/Feature is usable with minor issue

Steps to Reproduce
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Reference the above opendev review to see an example of the files that need to change when adding a new registry.

Expected Behavior
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It would be nice to have a less error-prone method of adding a new registry.

Actual Behavior
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Reference the above opendev review to see an example of the files that need to change when adding a new registry.

[Ghada Khalil (gkhalil)]

screening: code improvement. No functional impact on stx operations