Enabled write_only fields on bindings

[fao89]

https://pulp.plan.io/issues/6346
closes #6346

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[pulpbot]

Attached issue: https://pulp.plan.io/issues/6346

[fao89]

write_only fields started to be hidden due to this issue:
https://pulp.plan.io/issues/5622

(pulp) [vagrant@pulp3-source-fedora30 pulp_rpm]$ http :/pulp/api/v3/content/rpm/repo_metadata_
files/
HTTP/1.1 200 OK
Allow: GET, HEAD, OPTIONS
Connection: keep-alive
Content-Length: 468
Content-Type: application/json
Date: Thu, 26 Mar 2020 21:47:00 GMT
Server: nginx/1.16.1
Vary: Accept, Cookie
X-Frame-Options: SAMEORIGIN

{
    "count": 1,
    "next": null,
    "previous": null,
    "results": [
        {
            "artifact": "/pulp/api/v3/artifacts/b6514ffb-5352-463b-912a-fe0e79ed2371/",
            "checksum": "521f322f05f9802f2438d8bb7d97558c64ff3ff74c03322d77787ade9152d8bb",
            "checksum_type": "sha256",
            "data_type": "group",
            "pulp_created": "2020-03-26T21:46:27.812444Z",
            "pulp_href": "/pulp/api/v3/content/rpm/repo_metadata_files/5a47cc9a-2b5a-46e6-9cd1-dd7d129035cc/",
            "sha256": "521f322f05f9802f2438d8bb7d97558c64ff3ff74c03322d77787ade9152d8bb"
        }
    ]
}

current doc matches with what you see on DRF browsable and httpie: https://pulp-rpm.readthedocs.io/en/latest/restapi.html#operation/content_rpm_repo_metadata_files_list

with this PR, doc would return:

{
  "count": 0,
  "next": "http://example.com",
  "previous": "http://example.com",
  "results": [
    {
      "pulp_href": "http://example.com",
      "pulp_created": "2020-03-26T21:32:34Z",
      "artifact": "http://example.com",
      "relative_path": "string",
      "file": "string",
      "repository": "http://example.com",
      "data_type": "string",
      "checksum_type": "string",
      "checksum": "string",
      "sha256": "string"
    }
  ]
}

which has: repository, file and relative_path, that user will never see DRF browser/httpie response. This could mislead the users, as it did on the issue I mentioned.

[fao89]

not hiding write_only fields makes rest api docs diverge from DRF browsable API / httpie responses:

(pulp) [vagrant@pulp3-source-fedora30 pulp_rpm]$ http :/pulp/api/v3/remotes/rpm/rpm/8aab58c0-0b9f-4c99-9f13-8c1080831f42/
HTTP/1.1 200 OK
Allow: GET, PUT, PATCH, DELETE, HEAD, OPTIONS
Connection: keep-alive
Content-Length: 422
Content-Type: application/json
Date: Thu, 26 Mar 2020 21:49:51 GMT
Server: nginx/1.16.1
Vary: Accept, Cookie
X-Frame-Options: SAMEORIGIN

{
    "ca_cert": null,
    "client_cert": null,
    "client_key": null,
    "download_concurrency": 20,
    "name": "53ac1678-bebf-45eb-9dab-a5cebe849fe7",
    "policy": "on_demand",
    "proxy_url": null,
    "pulp_created": "2020-03-26T21:46:06.729011Z",
    "pulp_href": "/pulp/api/v3/remotes/rpm/rpm/8aab58c0-0b9f-4c99-9f13-8c1080831f42/",
    "pulp_last_updated": "2020-03-26T21:46:06.729027Z",
    "tls_validation": true,
    "url": "http://mirror.linux.duke.edu/pub/centos/7/os/x86_64/"
}

with that in mind I vote to reopen: #600

[dkliban]

@fabricio-aguiar Let's close this PR in favor of #600.

[fao89]

Closing in favor of #600