Container Support

The behaviour is taken from dockerhub. By default all registry repos will be public. If the user want to have his registry repo to be private he will need to create it in advance in pulp. Since distributions are the ones which directly interact with push/pull operations - container distribution should be create ahead and marked private. We should add boolean on the distribution model private and it's default value unless specified will be false.

If 'push' operation will end up creating the registry repo( aka- create a container distribution because there is not an existing one), it will be visible by default. This operation will also subsequently create a container push-repo