Count IPA as a login plugin when checking for enabled plugins
    
    The installer ensures that at least one login plugin is enabled.
    It didn't consider IPA, which automatically enables gssapi,
    when doing this calculation. Add a check so that IPA counts as well.
    
    https://fedorahosted.org/ipsilon/ticket/152
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>