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.
@JacobCallahan urlparse expect that the scheme is passed otherwise it will populate the `path`. This should be some detail described by the RFC 2396 which `urlsplit` and `urlparse` follows.

```python
>>> urlparse.urlsplit('someserver.test.com')
SplitResult(scheme='', netloc='', path='someserver.test.com', query='', fragment='')
```

Its power will be better used when asking for the proxy configuration using one env var only.

[1] https://docs.python.org/2/library/urlparse.html#urlparse.urlsplit