Migration Plugin

We should add to the docs an example of a migration plan and explain the purpose of each field and how Migration plan works based on the provided information.

  {
    "plugins": [
       { 
         "type": "iso",  # it should be a pulp2 type
         "repositories" :  [
           {
             "name": "foo",
             "pulp2_importer_repository_id":  "my_repo_id to use to find an importer to migrate" 
             "repository_versions": [
                 {
                   "pulp2_repository_id": "idA",
                   "distributor_ids":  ["pulp2_distributor_repository_id1", "pulp2_distributor_repository_id2"] 

                  },                
                  {
                    "pulp2_repository_id": "idB",
                    "distributor_ids":  ["pulp2_distributor_repository_id3"]
                  },
               ]
            }
        ]
     },
     {
       "type": "docker",
     }
  ]
}