Changed implementation of proxy setup

Changed the setup_http_proxy method to setup_proxy.
The same arguments are used as before. This change includes
workarounds for pulp proxy configuration. However, until BZ1114083
is fixed, easier proxy support will not be possible.
Add some preceding whitespace to make linters shut up about indentation issues.

```python
'{{'
'    "proxy_host": "http://{0}", '
'}}'
```

Or just use `json.dumps`. ;)