Docker Support

As a result of this story the Docker rsync distributor will perform incremental publishes when the following criteria are met:

The repository has been published with the Docker rsync distributor before
Units have been added to the repository(and published by postdistributor) since the last publish by the Docker rsync distributor
Units have NOT been removed from the repository since the last publish by the Docker rsync distributor

The user will also be able to specify a 'force_full' distributor config. When 'force_full' is true, the Docker rsync distributor will perform a full publish. The default value will be False( this will be done in platform)