Description of problem:

The _href should point to an actual endpoint, not a UID.

Steps to Reproduce:
1. Add a distributor to a repo_group

 curl -s -H Accept: application/json -H "WebFrameworkSwitch: webpy" -X POST -d '{"distributor_type_id": "group_export_distributor", "distributor_config": {"http": true, "https": false}}' -k -u admin:admin https://localhost/pulp/api/v2/repo_groups/test/distributors/
 

Actual results:

[
    {
        "_href": "a68bca8a-7cbb-4ec2-9bb6-8386447b55dd",
        "_id": {
            "$oid": "54e630a3e138231230a80605"
        },
        "_ns": "repo_group_distributors",
        "config": {
            "http": true,
            "https": false
        },
        "distributor_type_id": "group_export_distributor",
        "id": "a68bca8a-7cbb-4ec2-9bb6-8386447b55dd",
        "last_publish": null,
        "repo_group_id": "test",
        "scratchpad": null
    }
]


Expected results:

[
    {
        "_id": {
            "$oid": "54e630a3e138231230a80605"
        },
        "_ns": "repo_group_distributors",
        "config": {
            "http": true,
            "https": false
        },
        "distributor_type_id": "group_export_distributor",
        "href": "/pulp/api/v2/repo_groups/test/distributors/a68bca8a-7cbb-4ec2-9bb6-8386447b55dd/",
        "id": "a68bca8a-7cbb-4ec2-9bb6-8386447b55dd",
        "last_publish": null,
        "repo_group_id": "test",
        "scratchpad": null
    }
]