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. Example usage below.

PROXY_INFO=proxy://admin:p@$$w0rd@my.proxy.redhat.com:8888 fab -H some.random.machine.redhat.com -u root -p 'myP@$$w0rd1$$0c00l' setup_proxy