Fix handling of --arch param when a target is specified
    
    Without this patch mock-config ignores --arch option
    and generates invalid mock config files which contain
    incorrect statements like:
    
    config_optstarget_arch? = None config_optsyum.conf? = '..SNIP..baseurl=URL_WITH_ARCH_NONE..SNIP..'
    
    Resolves:
    * https://fedorahosted.org/koji/ticket/301
    * https://bugzilla.redhat.com/show_bug.cgi?id=1248368
    * https://bugzilla.redhat.com/show_bug.cgi?id=1161610
    
    Signed-off-by: Tomas Mlcoch <tmlcoch@redhat.com>