Migration Plugin

Create the following models to pre-migrate all the needed data.
Check the field types in pulp2 and in pulp3 before deciding which to choose.

Pulp2Repository
  - pulp2_repo_id
  - pulp2_object_id (it's needed in case a repo has been removed and then a new one created with the same repo_id)
  - pulp2_last_unit_added
  - pulp2_last_unit_removed
  - pulp3_repository_version

Pulp2RepoContent
  - repo_id
  - content_type
  - unit_id

Pulp2Importer
  - Pulp2Repository foreign key
  - pulp2_importer_type_id
  - pulp2_config (json) - plugin will figure out what to do
  - pulp2_last_updated
  - pulp3_remote

Pulp2Distributor
  - Pulp2Repository foreign key
  - pulp2_distributor_id
  - pulp2_distributor_type_id
  - pulp2_config (json) - plugin will figure out what to do
  - pulp2_auto_publish
  - pulp2_last_updated
  - pulp3_publication
  - pulp3_distribution