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.
I know, but I feel it makes the code more readable when you use the indices to inject more than one thing at a time. I decided not to replace every other instance of multiple formatting, in this file, to keep it logically separated from the purpose of this commit.