Move the Fedora Proxy Client to on_enable
    
    The configuration of base_url is unavailable until on_enable, and setting
    this on get_tree makes it always use the defaults.
    
    Merges: #171
    Signed-off-by: Patrick Uiterwijk <puiterwijk@redhat.com>
    Reviewed-by: Howard Johnson <merlin@merlinthp.org>