Fix '--signing-service' option not allowing HREF

I get the following error trying to use an HREF with the `pulp deb publication` command.

```bash
(pulp) [root@localhost ~]# pulp deb publication create --repository apt-hashicorp-bionic --structured true --signing-service "/pulp/api/v3/signing-services/01ecc724-c031-486d-955d-70986fdda1ea/"
Error: The option signing_service does not support href.
```