Refs #5639: Adds proxy options for Pulp plugins.

For all of these importer.json files everything except the port needs to be wrapped in quotes otherwise it doesn't generate valid json.  Also there are no commas.  Here's an example of what was generated:

```
{

    "proxy_host": quartet.usersys.redhat.com
    

    "proxy_port": 12345
    

    "proxy_username": admin
    

    "proxy_password": changeme
    
}
```