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.
@Ichimonji10 using the `-n` the output is identical

```
$ ping -c 1 -n localhost | grep icmp_seq
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.039 ms
```