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.
Should some if this code only be executed if BZ 1114083 is open? In that case, you can conditionally execute code with `bz_bug_is_open` from `robottelo.common.decorators`.