IPA: inherit ldap_user_extra_attrs to AD subdomains
    
    Currently the component of the IPA provider which reads the AD user and
    group attributes in ipa-server-mode uses  default settings for the LDAP
    related attributes. As a result even if ldap_user_extra_attrs is defined
    in sssd.conf no extra attributes are read from AD.
    
    With the patch the value if ldap_user_extra_attrs is inherited to the AD
    subdomains to allow them to read extra attributes as well.
    
    Related to https://fedorahosted.org/sssd/ticket/2464
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>