Story #3821: As a user, I can migrate from Pulp 2 to Pulp 3
Let plugin migrate all its content models at once and not one by one
Plugins like Docker and RPM need to process multiple content types at once because they are related.
Currently the migration plugin pre-migrates and then migrates content type by type.
To give plugins more flexibility, the migration plugin should:
- separate pre-migration and migration for content
- pre-migrate all the content before triggering any plugin specific content migration
- introduce an interface which plugin will define and the migration plugin will use to trigger plugin specific content migration
- adjust ISO content migration to use this interface.