Added some group distributor features and bindings
Merge branch 'pulp-2.2.1' into jcline-repo-group-publish
Added constants for the distributor dictionary keywords
added missing documentation and changed exception handling in create_and_configure_repo_group
made the path in RepoGroupDistributorAPI relative
Added the list of associated distributors to the server's group creation response
Merge pull request #536 from pulp/jcline-repo-group-publish

Added group distibutor bindings