Print a warning when enumeration is requested but disabled
    
    Add an explanatory message to be logged once, at the start-up,
    mentioning that in case enumeration is not enabled, getent passwd won't
    return all users by design.
    The debug level chosen to show the message is SSS_LOG_NOTICE.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/2301
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>