Container Support

We need to provide the ability to create/remove tag via pulp api and enable `docker tag` via docker client api.

Define separate endpoints:
POST /v3/docker/tag will accept repository or repository_version, tag name and manifest digest

call will return a task. If it is a new tag, create it and add to the repo, create repo version. In case it is an existing tag, remove the tag, create a new with newly provided digest, add it to the repo, create new repo version.

POST /v3/docker/untag ill accept repository or repository_version, tag name
call will return a task. It will remove the tag from repository and create a new repo version.