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.
Better to use `urlparse` module:

```python
>>> r = urlparse.urlsplit('https://proxyhost.example.com')
>>> r.hostname
'proxyhost.example.com'
>>> r = urlparse.urlsplit('http://proxyhost.example.com')
>>> r.hostname
'proxyhost.example.com'
```
Eventually the proxy information could be a single environment variable:

```python
>>> PROXY_URL = 'https://proxy_user:proxy_pass@proxy_server.example.com:8888'
>>> r = urlparse.urlsplit(PROXY_URL)
>>> r.username, r.password, r.hostname, r.port
('proxy_user', 'proxy_pass', 'proxy_server.example.com', 8888)
```