Migration Plugin

In katello we have some repos in pulp2 that do not have a feed associated with their improter.

If we try to leave these out, the migration plan will fail with:

["Provided Migration Plan format is invalid:'[\"'pulp2_importer_repository_id' is a required property\"]'"]}

If we try to include them, pulp3 will not 'pre-migrate' them as they don't have a feed url: https://github.com/pulp/pulp-2to3-migration/blob/master/pulp_2to3_migration/app/pre_migration.py#L322

Even though its missing a feed, and thus not pre-migrated, pulp3 will try to create a remote for it and fail (since its not premigrated).

I'm unclear what the actual fix is:
1) pulp2_importer_repository_id should not be required (its not needed)
2 pulp2_importer_repository_id should be required, but not actually migrated if it doesn't have a feed (if the importer is actually needed)