Migration Plugin

Add a pulp2 api endpoint that will return MP template in a json format based on the provided data.

This template is subject to further modifications unless whole plugin is intended to be migrated.

GET pulp/api/v2/migration-plan/?plugin=docker
Will return:

 {
    "plugins": [
       { 
         "type": "docker",
     }
  ]
}

GET pulp/api/v2/migration-plan/?plugin=docker&repositories=True

Will return:

 {
    "plugins": [
       { 
         "type": "docker",
         "repositories": [list of all repositories in Pulp2 for this plugin, one pulp2 repo is migrated into 1 pulp3 repo and repo version]
     }
  ]
}

GET pulp/api/v2/migration-plan/?plugin=docker,iso

Will return:

 {
    "plugins": [
       { 
         "type": "docker"
     },
       {
         "type": "iso"
     }
  ]
}

GET pulp/api/v2/migration-plan/?plugin=docker,iso&repositories=True
Will return:

 {
    "plugins": [
       { 
         "type": "docker",
         "repositories" :  [list of all repositories in Pulp2 for this plugin, one pulp2 repo is migrated into 1 pulp3 repo and repo version]
     },
       { 
         "type": "iso",
         "repositories" :  [list of all repositories in Pulp2 for this plugin, one pulp2 repo is migrated into 1 pulp3 repo and repo version]
     }
  ]
}

For further detailed specifications user will need to modify the json manually and also in the case when they don't want to migrate all repos. We need to provide docs that will explain how to do that.