Docker Support

Manifest model contains 2 fields name and tag which needs to be removed because they are misleading, we have a separate collection Tag to deal with that.

There can be situation when an image manifest can have several tags. Imagine the tags are : foo, bar, latest.

We would be able to save in DB just one record of the image manifest and it will have in the tag field 'foo'.
Meanwhile we will have 3 records in the Tag model.

When user will try into search for manifests with tag latest he would be able to query Manifest model and look at tag field which is lying. We even say so in docs, which is wrong, we would need to update the docs as well and encourage user to search through tags http://docs.pulpproject.org/plugins/pulp_docker/user-guide/recipes.html#tagging-a-manifest

1. remove tag and name field on the manifest model
2. write a migration which would remove those fields for present data in db