Migration Plugin

Docker Tag and Erratum are mutable types in pulp2.

1. add to the migrator pulp2 mutable content types
2. in pre-migration step, before saving Pulp2Content, filter out content that is mutable https://github.com/pulp/pulp-2to3-migration/blob/master/pulp_2to3_migration/app/pre_migration.py#L133
3. The immutable content will use bulk_create
4. Mutable content will be processed one by one and checked for incoming last_updated versus the one we have saved. Based on this the existing Pulp2Content with whether be deleted and created a new one or we'll catch and pass the integrity error
5. Flip the is_migrated flag to False on all the repos that contain the updated mutable content