Description of problem:

The `/plugins/distributors/<distributor_id>/` endpoint should return the distributor that matches the distributor_id.

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

Probably present in all currently supported versions of pulp.

How reproducible:

always

Steps to Reproduce:

1.  curl -H "Accept: application/json" -X GET -k -u admin:admin "https://localhost/pulp/api/v2/plugins/distributors/yum_distributor/"

Actual results:

{"display_name": "Pulp Nodes HTTP Distributor", "id": "nodes_http_distributor", "types": ["node"], "_href": "/pulp/api/v2/plugins/distributors/yum_distributor/"}%     

Expected results:

{"display_name": "Yum Distributor", "id": "yum_distributor", "types": ["rpm", "srpm", "drpm", "erratum", "package_group", "package_category", "distribution", "yum_repo_metadata_file"], "_href": "/pulp/api/v2/plugins/distributors/yum_distributor/"}%  

Additional info:

The Distributor controller creates a list of all distributors and simply returns the first of the list rather than matching the parameter.