ipa-server-mode: use lower-case user name for home dir
    
    In older IPA server versions where the AD users where looked up by
    winbind the user name component of the home directory path was always
    lower case.  This still holds for IPA clients as well. To avoid
    regression this patch makes the user name component lower case as well.
    
    Fixes https://fedorahosted.org/sssd/ticket/2263
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 48b1db73639135dd4a15ee153f958c912836c621)