SYSDB: sysdb_getnetgr returns ENOENT
    
    Replace call of ldb_search by sss_ldb_search to make sure that ENOENT is
    returned if no results were found.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/1991
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>