template visibility in subdomains

##### ISSUE TYPE
* Improvement Request

##### COMPONENT NAME

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UI
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##### CLOUDSTACK VERSION

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4.19.0.0
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##### SUMMARY

We are a school and use domains for different groups of classes. It would be good if there was an option for public templates/ISOs to be visible in subdomains as well.
We cannot find a way to display templates/ISOs in the parent domain and in the subdomains. With the setting share.public.templates.with.other.domains we can share templates with all domains or only with the current domain. However, there is no way to share the template only in the parent domain and in the subdomain. This would be a good extension function. With shared networks, for example, this is possible. With a parameter such as share.public.templates.with.sub.domains this could be achieved.
For example:
share.public.templates.with.other.domains=false
share.public.templates.with.sub.domains=true

Start by tracing the share.public.templates.with.other.domains setting and the template/ISO visibility handling, then compare it with shared-network behavior. Done means parent domains can expose templates and ISOs to their subdomains without exposing them to every domain, with the requested configuration documented and covered by tests.

Written by the indexing model from the issue text.