NSS: Fix memory leak in sss_setnetgrent
    
    struct nss_cmd_ctx was not released in function nss_cmd_setnetgrent_done
    and it wasn't used in the other function, because getnetgrent creates its own
    nss_cmd_ctx context. struct nss_cmd_ctx was released after closing client
    because it was allocated under client context. Memory leak is apparent with
    long living clients.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2170