Bug 1112859

Description of problem: When trying to auto publish a repo using the export distributor there is no way to configure it except for via the API. Pulp-admin allows for --export and --export_dir but those must be run interactively and cannot be scheduled/auto published

Version-Release number of selected component (if applicable): 2.3


How reproducible: always


Steps to Reproduce:
1. create repo
2. use pulp-admin --export will only run export one time and is not scheduled. Auto publish does not have an option in pulp-admin

Actual results: export_dir is not configured and not saved in the database. because export_dir is not saved this causes exports to publish an iso to http(s) instead of exporting the packages to the filesystem

Expected results: repo exported to the filesystem after sync (auto_publish)

Additional info: Using the API the setting is configurable with 
curl -E ~/pulp.pem -k -X PUT -d '{"distributor_config": {}, "delta": {"export_dir": "/repos/", "auto_publish": true }}' https://pulp/pulp/api/v2/repositories/rhel-7-workstation-optional-rpms-x86_64/distributors/export_distributor/

the request is accepted but upon retrieving the settings the export_dir is still not set

curl -E ~/pulp.pem -k -X GET https://pulp/pulp/api/v2/repositories/rhel-7-workstation-optional-rpms-x86_64/distributors/export_distributor/

{
    "_id": {
        "$oid": "53a9a36abbea5770c8d1cd42"
    }, 
    "_ns": "repo_distributors", 
    "auto_publish": true, 
    "config": {
        "http": false, 
        "https": false
    }, 
    "distributor_type_id": "export_distributor", 
    "id": "export_distributor", 
    "last_publish": null, 
    "repo_id": "rhel-7-workstation-optional-rpms-x86_64", 
    "scheduled_publishes": [], 
    "scratchpad": null
}