Migration Plugin

Currently if i have an empty repository in my migration plan, it is not migrated. We need to ensure that all yum repos are migrated, even if they are empty. From IRC:

ipanova: well a user has some repository in katello, maybe they just created it, maybe its been around for a while
but for whatever reason they haven't done anything with it yet
some client might be subscribed to it
its not necessarily an intentional use case
but a possibility
and all repos, empty or not, need to be available at the same path in pulp3 as they were in pulp2
some yum client could be configured to use it and start erroring if it starts getting 404s