Migration Plugin

There are cases when a migrated repo doesn't need a remote and it should be handled.
Currently we require pulp2_importer_repository_id to be specified but we don't migrate importers without a feed.

There are 2 options:

1. make it optional and require migration plugin creator specify only importers which has a feed
    - cons: responsibility is on MP creator
    - pros: easier to spot a problem in migration plugin if MP is correct

-2. keep it required but handle the case when importer is not pre-migrated (for whatever reason, aka config is empty)
   - cons: migration plugin has no idea why the importer is not available - bug or intention
   - pros: no responsibility on MP creator.-

Option 1 met more support with a suggestion to explicitly state in the error message reasons why the importer can't be migrated.