Clean up proxy handling in yuminstall.py (#604137)
    
    Make it possible to add proxy to AnacondaYumRepo objects from an
    object with a proxy string and optionally username/password.
    
    repos with no associated config file do not inherit the proxy from
    the main yum.conf file, add the proxy setting passed from stage1
    to AnacondaYumRepo objects that we create.
    
    Resolves: rhbz#604137