Introduce pages["use_legacy_storage"] flag instead of pages["domain_source"]

In %14.0 we want to migrate all self-managed customers to the API and the new zip storage.

Originally we planned to just remove the support for domain_source parameter and always use API, but if anything goes wrong with it users may not have a proper workaround. So want to default to api starting from %14.0, but keep support for disk source for a few more milestones as this workaround.

If we just change the default to api, users may not notice this change because they may already have set disk manually and overridden the API.

So we want to:

• remove domain_source from omnibus
• replace it with another parameter like use_legacy_storage
• add a giant warning next to it saying "THIS WILL BE REMOVED IN 14.3, ONLY USE IF YOU'VE DISCOVERED SOME BUG WITH THE API CONFIGURATION, AND REPORT THIS BUG!" or something similar.

This way users will need to make a conscious decision about enabling this flag and will be warned. And it will allow us to push actual deletion of disk support a little further than 14.0.